This type of spam is solved through minimum fees for transacting very small amounts of bitcoin. If a user is willing to spend $10 to insert a URL in the block

This type of spam is solved through minimum fees for transacting very small amounts of bitcoin. If a user is willing to spend $10 to insert a URL in the block chain, there is no way to stop them without making bitcoin useless. The move to treat very small amounts of bitcoin as simply invalid is intended to reduce the bloat of the unspent address cache (called the UTXO). This is explained at the top of the linked page.