The problem with chat filters is they inevitably only work for a brief time, until the RMT spammers work out what words and phrases are being blocked, and then come up with creative ways around said filters or blocks.

For instance, a common trick used by RMT /tell spammers was in FFXI where any message with 'com' in the message would be blocked (as RMT messages usually have a web address to their website), so RMT would replace the 'o' in 'com' with an '@' and even inform the recipient what the url really was. Hence the same would only happen here - it's a zero-sum game, no matter what SE does the RMT will always find a way around it.