Stop doing that stuff? I can! (4,2,2)
I believe the answer is:
pack it in
'stop doing that' is the definition.
(instruction to stop doing something)
'stuff? i can' is the wordplay.
'stuff?' becomes 'pack' (packing is a kind of stuffing).
'can' becomes 'tin' (both are metal containers).
'pack'+'i'+'tin'='PACK IT IN'
(Other definitions for pack it in that I've seen before include "How will you get all that stuff into your suitcase" , "Quit" , "Give over" , "Stop working - or stuff suitcase?" , "Slangily, stop doing it - stuff suitcase" .)