Stop using computers? Don't be ridiculous! (4,3,2)
I believe the answer is:
come off it
'don't be ridiculous' is the definition.
(I've seen this before)
'stop using computers?' is the wordplay.
'stop using' becomes 'come off' (as in coming off drugs).
'computers?' becomes 'IT' (information technology).
'comeoff'+'it'='COME OFF IT!'
(Other definitions for come off it that I've seen before include "Stop being so stupid!" , "You're mad!" , "Don't be ridiculous!" , "Don't talk rubbish" , "People climbing dangerous structure may be told to" .)