With limiting the number of posts which Newbie(i think Jr.members need to be in this group) can post daily, it is necessary to get approval for each post before publishing. So it can't be published any post without approving from forum moderators. Yes there can be a problem it is a lot of job for forum admins and mods, but this can be facilitated if more members are included in the moderation.
How does it look? By default, moderators, admins and DT members can approve or deny publishing new posts from Newbie and Jr.members plus there is need to add for example +100 other verified members which can vote/approve post to be published.
So, all of the new posts must have 5 votes to be approved or 2/3 of quotas set. In this way, the ability to manipulate multi-accounters would be reduced. Multi accounts owner if they pass verification can't manipulate to publish their alternative accounts posts.
Allowed posts going to be published, denied going to trash. Accounts with 30 posts and 90% denied posts going to be banned(numbers and percents is just for example)
Verified spam-catcher members would be interested in moderate and vote for post-approval, maybe they can get some benefit here. Something like merit but can be named Karma points. this points can be used as a multiplier which can help them to improve his forum rank. Also, they have a limit how much votes can do it daily, then he can't improving rank only in that way. Verified users need to have 80% accuracy rate of his votes or will be removed from this list.
This would make it much more difficult for airdrops, ANN bumps, Bounty account farmers and I think from there comes the most of this shitty posts
Most of the SPAM is done in Altcoins & Bounty section by these users, there had been a case in which 1 person was having more than 50 accounts Jr-Member and spamming the forum for the sake of bounty.
But yea if you can limit the accounts ranging Newbie-Member from posting in Altcoin bounty section of the forum then it will definitely help to reduce the SPAM caused for the sake of bounties.