To delete a user click "Setup", "Users" select the user you wish to delete and click "Delete". Please note that you must first remove the user from any ring groups they have been assigned to. You can see if the user remains in any ring groups when you open the user panel. Ring groups will be shown at the bottom of the page
NB - Deleting a user DOES NOT remove the license assigned to the user. The license will become free and you may then assign it to another user. You will still be charged for the license unless you also remove the license.