I'm aware this was a rude way to go about it, but at the time my friend and I were both at our wits end.
When I was at college there was this particularly pushy religious group going around campus making a nuisance of themselves. Since this was a small, private engineering school, everyone knew who they were and tended to avoid the members, either individually or as a group.
Right around Halloween of that year, they started doing the door knocking routine. Which while annoying once, would've been fine if they accepted being told no. Unfortunately, it just made them pushier. Both my roomie and I had told them twice each that night that we weren't interested, we were happy with our religions and to go away. She was polite each time, I started off polite, but became rather short (but no foul or abusive language) with them.
Thinking they were just limiting themselves to our dorm, and it was too late to complain to the appropriate office, and security wouldn't do anything, I went to visit a friend of mine in his dorm after he confirmed they'd been there and left after getting heckled.
So I'm chilling in my friend's room, the two of us just chatting when there's a knock on the door.
Yes, the pushy group was back.
Without missing a beat, my friend happily exclaimed, "Hey Reika, the sacrifices we need to get our cult going in Great Cthulhu's eyes have arrived. It's a sign!"
I went along with him and the push folks took off.
The next morning I went to the office of the guy in charge of allowing the various groups. Before I could say anything, he asked if it was about Pushy Religious Group, when I said yes, he said that it was being dealt with.
I've since had similar issues, but it's always been at my apartment, so I know if I call the cops when they won't take no for an answer, the cops will come to deal with the pests. I'm ashamed to admit I'm not sure how else I would've handled it if I were still at college. Not answering the door didn't work since they kept coming back.
At least I didn't follow the advice of roomie's boyfriend, he wanted to fill his super soaker with ice water and spraying them.