Computers can be removed from your database through your configuration console, under Lansweeper\Scanned Computers. Select the computers you're after and hit the "Delete" button.
To set a computer to non-active manually, browse to its Lansweeper webpage and, in the Custom Fields tab, change its "State".
Cleanup options are also available to automatically remove or set to non-active obsolete computer entries. These options can be found in your configuration console under Scanning Servers\Your Server\Options. Cleanup options are executed when the Lansweeper service is started and subsequently every 24 hours.