There are similar threads like this, debating on the most humane ways to euthanize fish. there is no humane, guilt-free way to euthanize your pet, but sometimes its gotta be done.
2 of the best methods IMHO-
submersing them in clove oil,
or submersing them in EXTREMELY hot water.
Im not too keen on bagging up my fish and slamming them into a wall, or other insideous, barbaric ways of killing fish. I get no pleasure from smashing a fish to death, and for those who do smash fish to euthanize, I strongly advise you to seek other methods to put your fish down.