Code can be changed to disfavor the use, making it only about promoting with no rewards from buying votes. That's one way. Otherwise, STEEM needs to be in the hands of those who will vote in for the change. STEEM money rules and gets to decide what happens.
This is not a "free market", it is a market encoded by economic algorithms.
One can change the algos to minimise what is seen as detrimental to the whole economic system.
Code can be changed to disfavor the use, making it only about promoting with no rewards from buying votes. That's one way. Otherwise, STEEM needs to be in the hands of those who will vote in for the change. STEEM money rules and gets to decide what happens.