I have tested this on mobile and on desktop. After you’ve added a shared collection to your profile it’s stuck there.
For your own custom collections you can delete easily: Edit collection → click on the red trashcan. But for some bizarre reason for collections shared / from the community there is no delete option.
Did it used to be possible to delete collections? When I now try removing the shared collection from my profile, I get two warnings about deleting my progress. CM forum: Too easy to delete a custom collection?
A related topic in the forum:
Archive a collection so its no longer clogging up review lists. This is still valid despite the welcome addition of that “max number of reviews per collection per day” setting. CM forum: Archiving Collections?