As an Administrator at any level, you have the ability to 'remove' a user from your event. To do so, select the Users tile found on the Event Dashboard, and once in the Users index, select the user you wish to be removed by clicking on them. You will now find yourself in their account page - select Edit User and the option to remove the user will be found to the right of the page.
When selected, the user will have all access and permissions revoked relating to your event. However, any documents, discussions or comments created by the 'removed' user will remain in your event unless you choose to delete them individually.