I know what you're talking about! My son just turned 3, and he still wants it. (I'm not even producing ANY milk any more, and he still wants to nurse.)
When he turned 2 he was still doing the full night-nurse, and we were co-sleeping. We'd put him into his own bed at night, but he'd wake about 11:00 or 11:30 and come into our bed to nurse. Then he'd nurse again at 2 or 3:00 and do what I called a "monster feed" for about an hour starting around 4:30 or 5:00. Ack. He ate regular solids during the day; he just supplemented with breast milk at night (and BTW, I continued to take pre-natal vitamins, to make sure that we were both getting the nutrients we needed).
I was getting ready to need to night wean him, because I was going to start taking the train for 2 months and needed to get up at 4:30 a.m. to do it. He would cry every time: "Don't take a shower!" And cry hard the entire time, if he had waken. So I had to wean him.
I criss-crossed band-aids over my nipples and told him they were broken. This worked to help night-wean him, and might have worked altogether if I hadn't given in the next evening. I was so engorged and thought that if I only nursed him during the day it wouldn't be so bad. He was so happy when I let him, then seemed to understand when I couldn't at night (I'd re-applied the band-aids). He still came to bed with us every night, and still cried about me leaving the bed too early.
When the 2 month stint with the train was over and I went back to our regular sleeping habits, I tried another tack. (The band-aids didn't work anymore). I put a lighted digital clock that he could read on the nightstand, and told him he couldn't "eat" until the first number on the clock was a 5:00. There were lots of grumpy episodes, but that seemed to help me get more sleep.
La Leche League says "don't offer, don't refuse." I think that's pretty sound, though I refuse when it's inconvenient (during the day or out of the house) on a regular basis. I just get him a cup of water or try to distract him. He does ocasionally try to put his hand down my top, but I just remove it without paying attention and he stops.
Since he now only wants it for comfort (there's no milk after all) I do provide it sometimes. It really makes him happy. And as long as I'm not losing sleep, I don't worry.
I hope I've given you a couple of things you can use. BTW, this book was wonderful regarding infant/child nutrition: "Super Baby Food." Good luck!