To delete a user from the Server, select the Users section by clicking the Users button on the main button bar, find the user you want to delete from the list of users you are presented with and then click on that user. You will now be presented with the User Configuration window with your selected user's details showing in the window.
The top button in the User control window Tool Bar has a picture of
a user being crossed out
If you click on this button you will be asked if you are sure you want
to delete the user, if you click on OK, the user will be deleted. If you
delete a user their e-mail will also be deleted.