Deleting backups with Dropbox files

Hi,

I have a shared company Dropbox which I access through Finder. I do not have access to all of the folders in here, for security reasons. I have Time Machine backing up my files to an external drive, no problem. But when I try to delete old backups, I cannot delete the backup files which I don't have access to. They're now stuck in my trash, as I cannot permanently delete them, or "put back" these files to their original locations. This means my storage is filling up and I can't delete backups to create space.

How do I get rid of backup files which I don't have access to?

Many thanks