Reducing the Steempower delegations would be ideal however i think there will always be enough people who delegate SP to upvote bots if those bots are generating such great returns.
In my opinion the best way to stop upvote bots would be to reduce the demand for such bots by making it so that new content does just disapear right after it has been posted.
Maybe the postion in the hot and trending tabs should also depend a lot more on comments and userinteraction than on amount of $ upvoted.