You have to kill it. There are various humane methods for this. One that I recently read (and I'm not sure how well it works) is to place clove oil and water in a bowl. Place the fish in the bowl. He will fall asleep. (they use clove oil in surgeries for them- so I read) Leave him in the bowl for *trying to remember*...I think it was like a half hour, maybe longer. I'd leave him for a good long while. He'll die peacefully.
Or... you can cut the head off quickly with a sharp knife.
In the case of larger fish, some people reccomend putting them in a sack and thumping them hard on the concrete. This has never been my favorite and really only works for big fish.
You really have no other recourse as I see it anyway. These particular fish always do this once they get a little bigger. (in MY experience anyway) Sorry...
Get yourself a nice fish to eat your algae.