even though the quest does not end, you can mark it "inactive". that wont move it to the archive folder but it will grey it out and get rid of the map markers for it.
Although for withermore specifically, mine did not end either and his body is gathering dust after I smashed his soul jar. I was unable to take his spear out of wall since I have no strength characters (all-rogue party!)
Last edited by Ratblaster; 16/09/17 08:25 PM.