When able, I always feel best about feeding them off to other fishes. There's no ideal way to end a life. I've used clove oil, ice water, fridge to freezer, and even swung a net against a hard surface. All of them make me feel crappy. I got out of electric blue rams just because of the number of deformities I encountered. If I have fish large enough to swallow them that's my go-to. When I had the LFS I kept a "pet" alligator snapper just for dispatching larger specimens. Culling is a necessary downside of breeding (or even owning) large numbers of fish. I wish it wasn't.
Nothing good happens fast in an ecosystem.