> What's a R9K robot?
The use is somewhat context sensitive. Reading between the lines I'd guess you are thinking of turning on the R9k option on some board?
Anyway, the 'bot is supposed to scan a new post and compare it to older posts. If the content of the new post is too similar to older stuff, the user is temporarily banned and their post deleted. The point is improve the "quality" of discussion by keeping things new, or something.
It can also be referring to someone who frequents the R9k board, or is just acting like they do, etc.