I have tried deleting shows and movies from my Collection list through a 3rd party app, and using the web version using Android devices as well as a PC. Nothing works to remove them. It just spins for a while then stops.
Try clearing your data (don’t clear your profile image) on this page: Sign in to Trakt - Trakt
Try clearing your browser cache.
If none of that helps, you can contact @support and have a ticket started to help you. If you prefer email communications, try support@trakt.tv and they should be able to create a ticket for you moving forward.
As for the 3rd party app, you’ll have to contact their support team to troubleshoot why their app isn’t working.