Go to settings > Multi-User Access to create or delete a multiuser.
Read here if you want to know how to create a multi-user account.
In your dashboard you will find a list of multi-user accounts. On the right side you can click on the recycle bin to delete an account.
How do I delete a multi-user account?
Click on the 'rubbish bin' then on 'yes' to delete an user.
It is not possible to reactivate a user after deleting.
If you delete a multi-user, the user is switching to a customer account only. So the user is not completely deleted but hasn't access to your dashboard.