well if the players are selected randomly, chances are they would log off or cancel it somehow and try to get together again in the next match. so the only solutions i see is that they could only re-enter after a period of time and this time could be a random number, different for each player, or they pay a fee or after doing that X times, they loose part of the stake they used to sign in (assuming there is a minimum) or for each time they cancelled a match, on the next one, part of their earnings (if they have any, is burnt)
obviously this doesn't prevent it but it can minimize it.