So I was able to get access to the 'wrong' e-mail account, verified the account, saw my device listed there, and then deleted the device. All that seemed to work perfectly. (Thanks for the suggestion Jan). However, when I now go to register my device again (on my device), it gives me the same error message, that the device is already registered with a different account. So it appears as though the delete functionality on the web app isn't working properly. I suppose it's not a big deal to just use the 'wrong' e-mail address, but it would be nice to have the functionality working correctly.