That's why we make a small step only showing top 15 ads right now. Spammers need to pay a high transaction fee to keep it on the list. Spams could be a problem at the early stage, but in the long run, it would cost decent fees to keep your ad on the list.

As for the concern of blockchain bloat, the minimum transaction fee based on number of bytes is charged for all the transactions including ads. It might force other users to raise fee for their normal transactions though.