If you want to stop a user being able to access the dashboard, follow these 4 steps:
1) Log in to the dashboard as someone with manager access.
2) Select Users from the menu on the left.
3) Click the pen icon next to the user you wish to delete.
4) Click Delete User.
The user will no longer be able to log in to the dashboard.
note: you cannot undo this action - if you need to reinstate access, you will need to add the user again