If there is a clear scam accusation with ample proof and community consensus about the validity of the scam accusation, an attack ad can actually perform a public service by warning potential victims about a proven scammer or scam tactic. In this case, i feel an attack ad to be justified, i'd even say such an attack ad would add more value to the forum than a "regular" (commercial) ad.
If it's an ad attacking a person or company on unclear grounds, i personally feel the ad should be taken down.
Like i said: it's a grey area... Who decides when there is community consesus about a certain scam? In the end it'll always be Theymos' call.
In any case, an ad should never violate the law in the jurisdiction where the forum is hosted (i believe it's hosted in the US).
This time around it's this ad:
https://i.imgur.com/UDUXZds.png
In the weeks prior it was one by the same user:
https://bitcointalksearch.org/topic/m.46520165
If it's an ad to warn other members off scammers or a fraudulent group of people,then I totally support it as it'll reduce the possibility of falling prey to them
But like I said earlier,i need more info about this,to be able to scribble out a valid input
Thanks for bringing this up. I've edited the OP with some definitions.