Can I re-use initials used on a deleted user?

No.

Users have to be unique across all users on Glide even if they are deleted. This is because you can restore deleted users, for example, if they had been deleted accidentally.

To use these initials again you would want to restore the deleted user, change their initials (for example change BN to BNx) and then delete them again. Note that if you have data linked to your user then we would generally recommend setting users to inactive as opposed to deleting them, this way they will appear in dropdowns/reports looking at historical jobs.