Surely if we tackle it on the UI side, we disincentivise posting to the chain.
I'd like to see a 'minimum transaction required to leave memo' slider bar in our wallets.
I'd crank mine up to 0.5 SBD/0.5 STEEM, as would almost everyone else. The spammers would stop sending even miniscule amounts, as it all adds up, particularly if only a tiny fraction of new users have yet to exercise their slider bar.
I'm not totally across the tech, but barring specific transactions from landing on chain sounds really problematic.
RE: The Spam Problem - Some Statistics