I had the same problem with my son. He would go in pull-ups or hold it until he was constipated. Since I didn't want him to be constipated I'd give him a pull-up to go in, but as you know as they get older the mess gets worse, and I was tired of cleaning him up all the time. I figured he'd grow out of it but after he turned 4 nothing had changed. We tried taking away his toys and time outs, even rewards for going in the potty like stickers or a few M&Ms but nothing worked for long, so it was time for something different.
My husband and I would still give him the pull-up but would watch for the signs of him starting to go (he would hide in his room or in a corner and tell us to go away)and then we would quickly get him to the bathroom and make him sit on the potty until he went. It took a little while and sometimes we didn't catch him in time, but as he got more and more use to going on the potty he started doing it on his own. At first we had to sit in there with him to make sure he went, but when he realized that we were not going to give in he accepted the fact that he was going to go on the potty and that was that.
Good Luck
A.