I agree with this idea, and do not see a reason for which it shouldn't be implemented. This should, at least, be implemented for ICO's (although this would make it somewhat difficult, but it would still work).
It won't stop them. Let's say there is a project made to scam others then in order for them to post it here then they pay 1 bitcoin then proceed.
If it "won't stop them", then there is no reason not to add this. They'd at least be down 1 BTC for even attempting to run a *scam* and this would turn away the [insertWord]Coin spam.
I would agree this idea also because after that there would be less amount of scam coins and ico's thread but also there is another problem. This forum is free service, if wedo that, then another bitcoin forum will try to get opportunity by mentioning that such service is free on their forum. Yes, btctalk is greatest and most known forum but there are chanses (to my mind) that it will affect a forum with both (good and bad side).
On another hand, it's really good idea but let's wait and see opinion of theymos.