Chess works for lichess.org. They write that the have tests to check if a human plays and you get banned if you use a program. Might be difficult for the higher ELO ratings, but should be easy for lower ratings, like < 2000. Games which are not popular is another idea, because it is unlikely that a program exists for it (at for some time). E.g. don't think that one of my favorite chess variants, Atomic, has a good AI so far.