An AOO does not stop a move action. You need to trip the opponent to stop him. But you're asking how to stop without tripping? Hmm.
These conditions stop movement toward you: checked, cowering, dazed, fascinated, frightened, panicked, paralyzed, petrified, stunned. You can get weapons that cause those conditions. For example, in the DMG there is a "Mace of Terror" which has a Fear spell imbued into it. For about 32,000 gold you could do something similar to your reach weapon.
There is also a feat, if I recall correctly, that pushes back or knocks down opponents. I don't recall the name of it. It's in one of the "Complete" books. It might do what you want, but I don't remember the specifics, so we'll need to see if someone else has more details.