DEAUTHORIZATION Unable to deauthorise a bookseller-specific ID from my ADE

I have ADE but linked (i.e. authorised) to a specific e-bookseller from whom I successfully purchased and downloaded several e-books.  

I now want to borrow e-books from a library which requires ADE for downloading to, and reading on, my Kobo.  All my attempts at deauthorising my existing "authorised user" thru ADE Help tab have been unsuccessful........"unable to erase authorization". 

Adobe on-line chat has been quite unhelpful in trying to resolve the issue.

Any help to resolve this matter would be much appreciated.

Robert