I've done this, albeit only once. To a whole group of people at the same time though. Ohk, story time. It's November 1998 and I'm in high school. I've got a monthly meeting, which for reasons related to the group had to be held in a public place. We held the meeting at a mall and were on our way out when my friend and I were accosted by a proselytizing church group. We engaged them in a conversation in which my whole goal was to get them to explain their position to me but to do so in a way which made them question many fundamental assumptions they held. My position was that I didn't know, but that equally I did not believe they could either. The conversation just went on and on. It lasted probably an hour and a half. I thought it was an enjoyable discussion and less confrontational than many I had, but other than that didn't think much of it.

The next November we were back at the mall as we were still going to meetings. This time we were in the middle of the meeting when another one of my trollish friends ran into a church group out at the mall and dragged them over to our table. Though it wasn't a tenet of our group most of us weren't religious and were a bit argumentative, hence my mentioning my friend's trollish ways. So, predictably we got into a bit of a contentious conversation which lasted well over an hour. Finally when one of their group started having a bit of a breakdown another said "wait a minute wait a minute.... were you guys here last year? We sent a youth group out from our church and not a single one returned." My friend and I had through an hour and a half conversation deconverted an entire church group which that church sent out to the mall yearly just before Christmas to proselytize on their behalf. I'm not sure exactly what that group did, I don't know if they took up some other flavor of Christianity, but I know my conversation with them had a large effect.