From the Users & Teams page select the user you want to suspend. You will see options to 'Edit' or 'Suspend' the user. If you click on 'Suspend user', the users access will be revoked and they will be removed from any adverts and email updates.
Any suspensions can be over-turned at a later date if needed.
Unfortunately we cannot delete users. But if you have created a duplicate user please contact us and we can help: on 01732 617460 or firstname.lastname@example.org