If you've joined someone else's library, you can leave it by clicking the Settings cog in the toolbar.
You'll see something like this:
Remove a student from your library
If you are the owner or co-teacher of a library, you can also remove students (or any other authors in the library). View the list of users in the library, fine the user and click on Actions > Remove user. This will remove them from the library and delete any books in the library that they created.
