Scooping

[Rew]

Oppenent picks up their cards during his/her turn, they didn't attack, end their turn or anything else that could lead them to winning. If they do this can I call a judge and end up winning a match? They didn't win and they picked up their cards. It's mean but it teaches them.

 

[PokePop]

Call a judge over.
We can't give one ruling for all the shades of difference this question could have.

 

[bulbasnore]

Pokepop is right => call a judge.

Two things every player should note:

1. Picking up your cards before both players agree on who won is a severe error in the penalty guidelines -- starting penalty for competitive events is a game loss.
2. Use of dubious game actions that cause an opponent to make a misplay is a possible Cheating violation, e.g. "I guess you won"; *opponent scoops*; "Oh wait, you didn't win. Judge!" -- starting penalty is disqualification from the event.

Regarding #1 -- like many game actions I advise you to be very deliberate. I'll shake hands and be sure of victory before I scoop my cards.

Its very sad to get a game loss because you're in a hurry and you don't want to be prey to someone who is good at 'titlting' the match.