Eliminating posting requirements is not a good solution as it has a bad effect on the bounty campaign promotion itself
Yes, it's the posts with poor qualities should be eliminated.
What I see is to have a more disciplined bounty manager as not all managers are capable of moderating all the people that are accepted in their campaign. If we want to see less spam this managers must boot out the spammers in their campaign and they can do it by actively seeing who is posting well or not.
Maybe requiring bounty managers to only have a limit of campaign participants upto what he/she or his/her team can manage to monitor closely. While participants will require to not leave until the campaign is closed, so everyone will still have a chance to join new campaigns.