Another suggestion is to push all the promoted posts whether by Steemit or voting bots to the "Promoted" section. That way the "Trending" section will stay clean all the time :)
This is something I think may work the best. We publicly acknowledged that the bots are a legit marketing tool and remove the stigma from using them
One problem I see with the first proposal though (5k SP limit) is that big accounts would just create a bunch of proxy accounts with the SP under the limit, and that way circumvent the restriction.
Yes you are right. They will find solutions to use bots. But promoted posts should be in the promoted section, because thats where they belong.