Malcontented guys need to be warned, only once, then reported to the fleet manager for his decision...which in many cases is demotion, or termination.
The fight deck is run by only one guy (or gal)...the Captain.
If those in the RHS have something constructive to say, fine.
Sarcastic comments have absolutely no place on the flight deck...especially from the co-pilot side.
Likewise for the Captain.
Treat the guy (gal) in the RHS with respect.
If that doesn't work...see above.