I don't think it will reduce the level of spamming. Jr members can be removed from signature campaigns, member should be accepted.
If we really care about this issue, why don't we start restriction while we manage campaign. As a bounty manager, you can put restriction. For example, if we do have a look on Service(Bitcoin), most of the campaign are limited to full member, in some cases, it's limited to sr member.
However, we also can limit with merits. For example, members can participate if he/she has 50+ merits, full member- 150 merits and thus. If we require only full member for participating in campaign, result will be same cause we have a huge amount of full member by default.