Please note you must have access to the staff records to be able to do this, if you don't, a manager on site will need to delete the user.
To mark a staff record as 'Deleted', you'll need to go into Set up on the main Navigator menu, then to staff records, enter the staff number or select the user from the drop down menu and click load.
Once you have the staff record loaded up, go to the Leaving details tab and enter a date left. You can also replace the staff member with another user, this will move enquires etc along to the 'Replaced by member'
After this, save the record and the staff member will marked as left/deleted.