Dig a hole and drop him in it.

https://steamcommunity.com/sharedfiles/filedetails/?id=2546042991

A troll above ground takes at least two or three days to respawn, usually longer with the RNG. Trolls in caves don't respawn in my experience. I will say though that if I have killed a troll in a cave and looted the cave thoroughly, I don't go back in. Perhaps they do respawn in caves.

As far as I know, and I have more than 1000 hours in the game, trolls do not respawn in caves. The spawn point in a cave only produces one troll. This makes cleared troll caves a real treasure for explorers, since you can build a campfire inside -- the ceiling is high enough so that you don't get smoked out -- and set up a cooking rack. Nothing else, unfortunately, since the cave is classed as a dungeon and you can't build a workbench, but you can put your bench and bed and another fire just outside behind a wall and gate and have a very nice and quickly built small fort to spend the night in.

GunsForBucks' advice was good, too. Dig. You can dig all the way under the Elder's altar and set up a base there if you want. But even a few trenches in the right place will do wonders if you're having problems with trolls or Stone Golems.