Error - 'This user is a Access Evo user so cannot be deleted' When Trying to Delete a User

The best course of action when dealing with leavers in Dimensions is to create a security profile with zero permissions and move the leaver into this profile.
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You can't delete users that have been linked to Access Evo in Dimensions. You also need to make sure that if they're a Microsoft Windows user, you need to clear the Windows Authentication box.