I wasn't 100 per cent sure what you meant by this so I looked it up and found:
To post a deliberately erroneous or antagonistic message on a newsgroup or similar forum with the intention of eliciting a hostile or corrective response. Also trans.: to elicit such a response from (a person); to post messages of this type to (a newsgroup, etc.).
but there was also another meaning involving fishing for business, i.e. hawking one's services.
I assume it's the first one you mean. Is it really a big problem on this site? If it does arise, you're right: the answer must be to ignore the diversion and continue the genuine discussion.
I have a feeling the troll you're referring to is actually of the latter category. If you follow the link he/she has posted on a number of replies it goes to a website which is advertising a book.
The trouble with raising the subject is it acts like blood in the water and sooner or later more turn up.
If that happens simply keep quiet and ignore them and they'll swim off to better feeding grounds.
Yes, I'm rather intrigued as well. I think the general rule with these types is Do Not Feed the Trolls. They only want attention and if you're not giving them that, they tend to slink away. A couple of times, I've posted links to this and made no further reference to them... worked like a treat.
I didn't actually mean to get people intrigued and discussing it. I just didn't want us to stop threads because of a suspected troll. To me, that's like giving into them, really.