Opening doors

My 1 1/2 yr old male cat, Tony, has learned how to twist a door knob, and push it open. I got a new knob with locks, and now have to lock it every time I leave upstairs. I live on the second level of my paretns house, and they have three cats down there. I dont want the cats mingling, because the two eldest had enough problems when I brought home my college cat. The problem now is that he wont stop slamming on the door and jiggling the knob. It gets very annoying, esp in the middle of the night. He also stands at the door yowling. He has seen the other cats and gotten into at least one scuffle. I just dont know how to break him of this behavior. :eek:

Try a squirt gun. Every time he becomes a nuisance, squirt him. That usually helps. Usually. With every cat but mine. :p

Blessings,
Mary