Blocking lines completely can be troublesome because of the unintended consequences blocking words like "recruiting" and whatnot might bring. I'd suggest making the trade chat a separate window and placing it behind the general chat. The recruitment spam is annoying, indeed, but I find the "Casino now open. 66-99 double gold back. 100, triple gold back. No scam. Minimum 100g" /yell spam in Stormwind much more annoying.
This is how I've set up my chat: http://imgur.com/99TXQ
Yeah, yeah, laugh at the "Friends: 0" thing. I hate you.
Trade chat is only visible if I choose it to be, and even then it's still spam throttled. Curse is down right now, so I can't really find a true spam blocker, but unless you actually use the trade chat, there's no reason to bother with the headache. Just remove trade chat from view if you're annoyed by the constantly scrolling wall of text there.
Edit: Personally, I find the casino spam in /yell more annoying than the trade chat spam. I've had to move to the Dwarven District in Stormwind because of it. Fortunately, advertising casinos/gambling isn't allowed by Blizzard
, so I report those. Edited, Dec 12th 2012 1:10pm by Mazra