Locked into outdated default Adobe ID

I've inherited my mum's old computer. When I downlaoded Adobe Digital Editions for the first time today, it automatically signed me into her old account which is now inactive. I can't sign out since it's the default account, but there is no password because the account is inactive. I've uninstalled and reinstalled the app to no avail. I can't erase the authorisation or anything.

How can I sign out of a default account that seems to no longer exist except on my computer?