Isn't there? It's not guaranteed, but if there is increasing competition for block space then the spam will have an increasing price card attached it. The more valuable the transaction is to the sender, the more he's willing to pay in fees. Assuming the spam is not really of any long term value to anyone, I'd say it's quite likely that less and less people will be willing to pay the fees.

If it does not get priced out over time, then apparently it is valuable enough to enough people. In that case I'd say it's fair to ask "who am I to declare it spam?"