I don't see what the problem is with smashing it up against a wall. The fish is in pain, and a good whack is gonna put it out faster than anything else. With clove oil, the fish will be oblivious to what is going on, so yes, it is the best method, but a good impact with a wall will be the fastest. Fish aren't quite as advanced organisms as we are, and I'm not certain, but I believe that their central nervous systems aren't completely concentrated in their heads. What I'm saying here is that severing its head may not kill it 100% immediately, it may just put it through a few seconds of excruciating pain before it dies.
In my opinion, you've got 2 choices:
1. Smash it
2. Use a concentrated solution of clove oil
I am 99% against euthanizing fish, but I'd say your case falls in that minute 1% that I am in favor of it.