After defeating Dagoth Ur there are still Ash Beasts and Corprus Victims around that will respawn some days after their dead. Is there a way to ultimately "free" them? Maybe a mod if not natively?
As far as i know you cannot ultimately free them in the original game or its official expansions. They will still spawn even after defeating Dagoth Ur. I dont know of any mods which would handle this issue.
For more info you might want to check out this great FAQ: http://faqs.ign.com/articles/497/497229p1.html