The best way to eliminate waste is to increase a price. Each transaction should have fee growing with increasing number of transactions. If you had to pay 0.1 SBD instead 0.001 SBD for each transaction, it would too costly for spammers to abuse the system.
Spam in comments should be solved by shareable individual blacklists in UI. Most people would end up using same spam filter derived from witnesses or thrusted people. And since most spam would be filtered, the spammers would stop posting because they would have no interactions. If that didn't work, comments could cost a fee.
Both fees should end in reward pool.