Sock and hammer: insert fish into sock, rag, paper towel, etc. and hammer hard, then throw whole thing out or wash sock/rag if used
Decapitation: use plant shears or cutters, you can even use scissors, and slice off the fish's head, disposing of the body
Euthanization: kill fish with overdose of fungus medicine or something similar
Reuse: feed fish to other animals or bigger fish
Quick killing: either snap the fish's spine or crush their brain case
These are different ways to cull, not all desirable. Some may say that culling fish for aggressiveness is cruel, but what will you do? Rehome them to someone else's tank where they will certainly wreak havoc? Let them kill your helpless, passive fish? Keep them solely and let them take over your tank? It is time to take action when fish come out of line.