A name is on your watchlist because you're on the trusted list for the person's WikiTree profile. Remove yourself from the trusted list (the privacy tab on a profile) & the name's gone from your trusted list (I would recommend you stay on the trusted list if it's a person who might be in your tree and the profile's not open).
If your question was how to remove a lot at a time... you can do "bulk trusted list changes." On your Watchlist page, click "more info" in the first line. Then click the "in bulk" link at the end of the second paragraph. That will take you to the page that will walk you through making bulk changes to your trusted list.