Authorizing and deauthorizing new Kobo?

I have bought a new Kobo and cannot authorize it because an older email and account exist. I cannot deauthorize the old one despite numerous attempts over several days. I contacted the help line, they changed my old id email address to the new one and new password etc. but I still cannot deauthorize the old account. I was assured that my new ID works and it does to access my new account but I still cannot authorize my new Kobo. They suggested I post my question on this forum. I hope someone has the answers.