If you are the Account Owner of your account you have permission to make changes such as adding or removing users.
How to remove or delete a user
Log into your account.
Select My Account > Users.
Select 'Edit for the user you wish to remove/delete.
Select the red 'Delete' button.
Save.
Useful Information
Although the user has been deleted, you can still find them on your account and reinstate them if needed.
Select My Account > Users.
Click on the drop-down list to filter to view 'Deleted Only' users.
Select Edit for user followed by the Undelete button if you wish to reinstate them.