Impact is the fastest and most direct method. Crushing the head with a paver or brick is ideal, swinging the net can work too.
Some people use an in-sink garbage disposal (very quick but horrible to think about)
Clove oil works fairly slowly and you really need a decent amount to actually kill a fish. Smaller amounts will render them unconscious though, and then they can be dealt with in any way you see fit.
Personally, I would drop the fish into a bowl of ice water, as cold as you can get. This instantly shock will KO the fish and then you can deal with them in a manner of your choosing. Leaving the fish in contact with the ice water will eventually kill it.