Several times, in fact.
The first time was when I was in grade 8, when I was re-united with an old friend from one of my elementary schools. His parents had heard about "some kid when went crazy and killed someone from playing D&D." I was a quiet, reasonably well-behaved kid who did well in school, and never got into fights, didn't drink, etc. But because I played D&D, he wasn't allowed to come over to my place, because his parents were convinced I was going to kill him. He wasn't even allowed to talk to me on the phone for more than 15 minutes, because they were afraid we'd play D&D over the phone. I'd even brought my D&D books over the one time I slept over at his place so they could see them and watch us play, but they were unconvinced. They seemed to have no problem with him hanging out with high-school dropouts who always got into fights and got drunk every weekend. Whatever.
The second time was also in grade 8. We'd started up a D&D club at school, and we only got to our second meeting when the principal came and told us that we weren't allowed to play at school. The mother of one of the grade 10 kids (this was actually that guy that introduced me to the game at Scout camp) had demanded that we not be allowed to play, basically because she felt her son was spending too much time with the game instead of school. So that ruined it for all of us. We tried reasoning with him, and my mother fought for our right to play, even asking if we could start a club if students brought signed notes from their parents. Still no go. So instead, we just left school at lunch to go and play at Yuri's house (he lived a few minutes away from the school). Good plan, guys. Encourage us to just leave the school grounds.
Later in grade 8, we got in trouble for bringing D&D books with us to play on the bus on our band trip (we were going from the BC interior into Alberta, so it was a long trip). We were told it wasn't allowed. Again.
I don't remember if it was the same year or not, but my band teacher saw us in the halls with D&D books, and told us that he had order to confiscate all D&D books, so he'd better not see us with them again. I would have liked to have seen him try that.
My grade 9 social studies teacher (who was a complete moron), at a parent-teacher night, told my mother that she should be concerned about me because I played D&D.
In grade 10, we'd just decided to not tell anybody that we were playing, and we technically weren't playing D&D anyway. It was a game of "Trandimensional TMNT" in the world of Conan. My friend Steve was "Arno the Agitated," a mutant ardvark with an Int score of 7. I was Xof, a mutant fox wizard. Anyway, the teacher whose room we played in didn't mind what we were doing, so we played in there every lunch hour. Then one day, there was a substitute teacher in there. The characters in the game were in a tavern, and Arno used his "Aquilonean Curses" skill -- which was his familiarity with local swear words -- to talk to the guys in the bar. The next day, that substitute was there again, and she was armed to the teeth with religious anti-D&D propaganda. She lectured us on the evils of D&D for the whole lunch hour, totally ignoring any arguments that we made. She talked about rescuing her sister who "dabbled" in D&D. She got off on some tangent about how she wouldn't let her children watch "Masters of the Universe" because it implied that God is not the master of the universe. She also wouldn't let her children watch The Smurfs, because the Smurfs used magic, and all magic is Satanic, whether it is white magic or black magic. Obviously, there was no reasoning with a person like this -- the Satanic Smurf Lady, as she became known to us. The next day, the original teacher was back, but the substitute had complained to the school administration, so we were once again told we couldn't play on the school grounds. So once again, we played at Yuri's house instead (he was the GM of that game anyway).
Grade 11 was at a different school (the senior high), but it was the same group of us. My house was 5 min from the school, and I had a huge 4' x 8' gaming table in the basement, so we just played there at lunch. One day, the vice-principal saw us with our multi-sided dice in the hallway, and (with a smirk, obvioulsy not really caring) told us "you're not supposed to be playing those games at school." As he walked away, I remember one of us saying "hey, let's go to Chris' and sacrifice small mammals." (I'm Chris, btw)
I think that was it, from my teenage years. That's quite enough already... Well, we got teased a lot from the jocks and the in-crowd, but that didn't stop us from playing.
Never really had any troubles after that. I'd gamed with a few people who didn't tell their parents they were playing, because they knew their parents would have religious objections to it.
Well, my ex-girlfriend (ex-fiancee actually), during the 5 years we were together, never understood why I liked to play D&D, and was always harassing me not to hang out with my D&D friends, because they were too geeky. And she thought it was a waste of time (same goes for my video games). I didn't play that often, and when I did, she would keep calling me every half hour after I'd been there for a couple hours, demanding when I was going to come home. When I say I didn't play that often, I mean it. I probably played D&D (or any other RPG) no more than 10 or 12 times during those five years, so it's not like I was neglecting her for the game or anything. And she'd always try to stop me from going, usually as I was about to head out the door to a game, without any consideration for the others who were going to be at the game. I'd had to cancel a few games I was DMing, on only a couple hours' notice, because of getting in a big fight with her about it.
So that's my story of people trying to stop me from playing D&D.