We limited it by making a only in the house or car rule. And then when his dentist decided it was time to work at getting it away altogether, this what we did:
We started with poking a hole in the paci. This flattened it when he would suck on it. That led to his chewing on them and creating more holes. He got annoyed with them and wouldn't suck on them, just put them on the shelf above his bed. When he asked for new ones, we told him that they don't make them for 4 year olds. (He was 2 days away from his fourth birthday.) He hasn't had one in weeks and seems fine with it.
This is a kid who had one almost constantly and has a baby sister that uses a paci. Never in my wildest dreams did I think it would be this easy, but I do think that the fact that it happened pretty much on his terms -- nothing forced -- made it that easy.
Good luck to you!!