Firstly, I see no problem with anybody following their religion in privacy, quietly and without dragging in people who want to keep real-life religion out of gameplay. Of course, in this situation, this guy's not doing that. He's being obnoxious about it and it should be stopped - this is encroaching upon the free will of others, it's not acknowledging their right to choose their own religion without being harrassed about it, and, well, it's breaking the immersion (which is hardly an issue in this case, but it's still an issue regardless.)
I am displeased about the posters in this thread claiming that the OP and his supporters hate Christians or wouldn't care if it was another religion preaching in public. I can only imagine that if these people - and I mean those who brush aside the OP's complaint with "You just hate Christianity!" - one day came across a llama full of communication crystals spamming, say, Islamic propaganda, they would be disgusted. Claims that Christians, of all groups, are unfairly persecuted always irritate me; I know as well as you do that you are only defending this behaviour because you think that because it's your religion, it's superior.
If the llama were spamming "Allahu akbaar," the Shema Yisra'el or even Hare Krishna chants, you would be the first to complain. And yes, I would support you as I'm supporting the OP now - and if I, say, started up a synagogue service in the bank, I would expect others to confront me about it and they would be well within their right to do so. Nobody of any faith has the right to enforce their preaching on others on common ground like an online game. You can say "They are just words," but the implications and ideas wrapped up in those words are what's important. To me, being proselytised at on UO is as bad as having to endure others' religious ranting in real life because while UO may not be real, the intentions of the ranters are exactly the same.
tl;dr: Kill the llamas.