Deleting user and then re-inviting

[Grade1]

I am trying to give my Accountant access to my online file but unfortunately I set them up as user with an incorrect role. I've since deleted them as a user but now can't invite them again as MYOB is telling me their email address is already in use by another User. The thing is they are no longer in my file as a user (including as inactive), and I can't use any other email address for them.  Any suggestions please?