How can I force trolls to drop their inventory before I kick or ban them?
Solution 1:
There is a full list of the commands on the wiki.
I think this one would to the job (killing him, hence dropping the precious items) : AllPlayers[number]:PushEvent('death')
number being, of course, the identifier of the troll, which you can determine from c_listallplayers().
Solution 2:
You can spawn something to kill him, grab all the stuff and then ban him. You get the stuff, the trolls get out of your server.
I am not really good with admin tools for Don't Starve Together though, so I can't post any commands for you, but just a simple idea.