You may delete a user by opening his/her profile (accessible through Administration => Users > Users ) and clicking on the “Delete” option in the right pane.
When a user has been active in an account for a long time, it may be very difficult to actually delete the user from the database. In such circumstances, we simply recommend you change the user’s “Status” to “Inactive – Access Denied”, in the “Access Rights” section of the user profile.
This solution allows you to keep all historical data associated with the user (including timesheet and expense data which are reflected in task and associated project statistics) yet hides the user from all lists for new assignments. Setting the user to “Inactive – Access Denied” also puts an end to all e-mail notifications being sent to the user.