For culling fry, the most common method I've heard of is putting them in a tank with a large fry munching fish as mentioned.
All other methods are more commonly used to euthanize sick fish, freezing and flushing being the most questionable methods.
Clove oil should be used with caution as it generally only knocks fish out, it's very difficult to overdose on clove oil enough to kill the fish. I use it for puffer beak trimming and the fish recover quite quickly when put into a bowl of clean tank water. It's also not a good idea to use on anabantoids (bettas, gouramis, etc.) as it will only cause them to struggle. If you do use clove oil, use it to knock the fish out, then finish the job by adding vodka or some other grain alcohol.