Deleting staff from Microsoft Bookings

I'm using MS Bookings to manage a space I work in. One of the things I did early on was make an error on a staff members details. I couldn't amend their details afterwards, so deleted their profile. Now when I try to add them back into the system I get a message that tells me "That Staff already exists".

I've checked my Staff list and they don't show there.

My question is:

How do I delete them to make their new profile? I can't use a different name to get past this issue either.