This is because he was created as a global myadmin user, and not with basic authentication issue. He will need to be changed/deleted through the SDK to fix this. To do so, login here: https://www.geotab.com/mygeotab/sdk/bulkEditTool.html with the same server/database information as the database you are working with. Follow the image below to change them to Basic Authentication. Click analyze and submit. If you did it right, you should receive no errors.
NOTE: This will work when someone tries to enter a user and it says a user with that name already exists. This is because they are a admin user thus, they cannot see them on the user list