There are a couple of other ways at the class list that may work.
First thing I would try is enabling the Old Homework Sets Editor for that course. It's possible that you might not get the error on that page, and be able to delete the offending records from that page.
The other thing to try is the Instructor Tools page. If that loads, then you can select the mangled users, and edit them from that page. Then you may be able to clean up any fields with weird characters, which might let you load the Classlist Editor.
On the database side, I believe it won't be too bad, since it's likely that no homework sets were assigned to these new users. If no sets have been assigned to the user, then you should just have to delete the offending records in [coursename]_user, [coursename]_permission and [coursename]_password.
At the very least, deleting the records from [coursename]_user should prevent them from displaying in the Classlist Editor, which should give you access to the page again. If you're worried about corrupted data at that point, then export all of the homework sets, delete and re-create the course, then import them.