To delete an existing user login to the MSPControl and click on Users (User Organizations),
or go to Organization > Organizations > Users.
You will see the list of all users. If you have many, it might be a good idea to use Search form in the right to find the one you need. You can choose whether you want to search by Display Name, E-mail Address, Domain Account, Account Number, or Login.
Once you found the user you were looking for, click the corresponding Delete icon on the right, and confirm your choice. Now this user has been deleted.