The idea is that you can reduce the sp of bidbots by a negative delegation. It probably should not be possible to go below 15 sp to not make this to abusable. Instead of flagging a post this would be like flagging a person. 

Then that bidbot would not be able to sell the vote anymore as the vote will not be worth much anymore. 

Even better, when all the big guys work together and use a part of their sp to target the biggest bots, then the honest voting sp will have a much bigger impact. The reward pool will stay the same and is now distributed by all the honest curators. 

I am not sure this would work out in practice, but it sounds like a good idea to have in addition to a flag also a negative delegation.