Genus Turpe (argument from humility) is also the rhetoric of choice since it is impossible to prove a negative. Your natural position is the defensive (which can of course lead to trouble too), but point this out: Your opponents have NOT proved that there is a relation, it is all speculation. There is no evidence for a relation between computer games and crime. There are criminals who have played computer games, just like there are criminals who have played hockey or read classic litterature.
Also make use of hyperbole: If someone thinks games should be outlawed "to be on the safe side", carry this argument further, perhaps using the Bible as an example.