In order to remove the user, you have to be logged in as an user with Admin permissions. Then you'll be able to deactivate and remove a particular user.
See below for details.
Go to Accounts management page
Go to Edit user details link
Click Deactivate option to deactivate the user
As soon as User account is deactivated you can activate it again or remove completely.
When you deactivate user, option to remove user will be visible.
If you choose to remove user then you have to select new owner of the presentations and other contents (e. g. Tags or Orbittour Templates) of the removed user.
The user account will be removed and all the contents from this user will be moved to the new owner.