At the bottom of the app, tap My Stuff, select your list, tap the (-)negative sign on the upper right and it will turn to (+) positive or plus sign then tap back to do the same with all the lists you want to remove.
For users using a computer browser
Simply tap the heart to unlist it.
Or go to the films one at a time and hit "Remove from Watchlist".
Otherwise, if you have lots on your list and you cannot find them, and you remember the title of the film or show you can search for the title, remove it from your list, then add it to your List again.