Is there a way to disconnect CiviCRM contacts and Drupal Users via the UI?
This is just a top-of-the-head thought and I am not sure I've thought through all the ramifications, but could you: 


Create a new contact (blank, essentially, or with the same name) 
Run a search that pulls both of these, and select to merge them
Flip the contacts so the old contact is considered the duplicate contact
Check the boxes to transfer all the info except the Drupal user to the new contact in the process of merging?


Test first, obviously!