Searched the forums and the rules and couldn't find anything that popped up that stems from a situation I dealt with today and felt this might be the most appropriate forum to post to.

I had two players get to their seats when rounds were posted and immediately begin play before the clock had been started. It was the final round in a small tournament at Regular level enforcement with 12 people, so I was able to easily see everyone. I asked both players to stop play immediately and reshuffle their decks and not start play until the clock began, and this upset one of the players greatly but it was left at that.
My question is how would this normally be handled? I could not find anything really mentioning this kind of situation.

At Regular REL I would have a talk with the player and explain to them that they can do pregame preparation like shuffling their deck, present and take the starting hand (and also mulligans) but as long as the HJ doesn't announce the start of the round they definitely need to wait.

Reshuffle is an option to ensure the two players have the exact same start then anyone else, in this situation I might consider to stop but don't force a restart if they didn't advance too far (like playing several turns compared to for example playing the first turn)

Since there isn't any official approach I feel like this is one of the things that falls under situative judgement.

i would consider it extremely poor customer service to force these players
to restart their match and i would never ask them to do that. the most i
would do is ask them to wait, but more realistically i would not expect
players to wait for the round to officially start at regular. imo there's
no need to stop them from beginning as soon as they find their opponent.

To clarify a bit more it was the second players 4th turn by the time I noticed play. From the time I asked them to reshuffle and when the clock was actually started was about six minutes.
My own reasoning behind the having them reshuffling is I felt it could be an unfair advantage. In a hypothetical situation if it had gone unnoticed and it had went to time between them, that would be 7-8 minutes of extra gameplay that they would have over those whom waited for the clock to start in order to determine a winner.

It's Regular REL and it's largely a matter of your own personal preference and experience as a HJ. Personally, I'd ask them to stop playing immediately and remind about always wait to the start of the round (they can shuffle before, and/or determine starting hand). But NEVER “restart” the game. That seems a horrible solution for me, and that's even worse if few turns have been played.

When players begin playing their match before the round clock starts, just approach the table and gently ask the players to stop until you get the round started. Educate the players for future matches that they are allowed to perform their pregame procedures beofre a round starts but not to begin playing until a tournament official gives the, “Go ahead!”

Also, having the players restart their game to make it “fair” for the rest of the tournament makes it seem like we are punishing those players for their error and that's not something we want to encourage especially if those players didn't know better.

Let those players keep playing that same game and kindly remind them to wait for you to start the clock for the next rounds they play.

I hope the extra 6 minute wait is atypical as that is quite a long time
after pairings have gone up (plus the time for those players to find their
table and shuffle and play 4 turns each). If that period of time between
pairings and round start is common you may want to look at your procedures
and player culture amongst other things.

More on topic: As others have said, restarting the game is bad for the
players, they now don't have a real game one as they know what each other
are playing before the start of game one. What would you do when one of the
players starts sideboarding before the real game one? There is nothing
stopping them from doing that.

Thank you for the responses. I owe both these players apologies.

And if this situation happens in competitive REL?

Ask the players to stop playing and wait for the start of the round to be announced.

That's what I thought (most of all because I believe the arguments Gareth Pye gave are really strong). But I was discussing it with another judges and we have different opinions on the matter. Thanks Gareth Tanner! :)

I'm just going to back up what other people have said here - asking the players to stop and wait for the round to start is a good solution; forcing them to shuffle up and restart is less good. It is a massive feel bad for the players (unless one had to mulligan to three or something, in which case they are unlikely to have had the time to start playing before you started the round), and additionally it will often ‘cost’ them more time after the start of the round than they ‘gained’ by playing a turn or so early in terms of having to shuffle and resolve mulligans anew.

If it would be unfair towards the other players to let these two have an additional 6 minutes, you could also ask them to pause for 6 minutes.

I'd just let them get on with it, but it's certainly possible to fabricate scenarios where 6 extra minutes translates to an actual leg up in the tournament standings, what with a draw being worth less than half a win.

I had something very similar happen at a SCG Open. I asked them to pause their gameplay until the round clock had begun, and then I told them to play on. I later ran this situation past an L3 and he confirmed that I handled it correctly. He told me that in the past the fix might have been to make the shuffle back up and start over, or make them hold for X minutes once the clock started (equal to how much time they played before the clock began), but none of these methods are good customer service.

If the event were Sealed Deck, how would you handle someone who ripped open his boosters prematurely?