If you really want to stop spam in the forum you should not let spammers join a signature campaign in the first place. You can only do it if all bounty campaign managers screen their participants before being part of the campaign also they should constantly monitor the quality of their post every payout to assure that they are good for the most part of the campaign. Unfortunately not all bounty campaign managers care about quality and they usually accept participants who apply first in the campaign in order to fill it up. Having an option to remove signature in your thread won't really solve anything cause I doubt a lot of members will be doing that.