these limitations you are proposing will actually make things a lot worse.
for example if someone was earning $10 and you limit it to $5 then they will create 2 accounts and now they have to spam twice as much. if you ban account buys it will just migrate to somewhere else and because of that ban (less supply) the price of such accounts shoot up so there will be more incentive to do account sells and more people would start doing it (farming accounts and selling them). the result is more spam, more merit abuse,...
It would happen before introduction of merit system. But now for random spammer it's almost impossible to build forum account from, or he have to put lot of efforts into it. Same goes with account sellers, I doubt that they have so many accounts farmed. It's questionable, how many of these account sellers in the marketplace really holds accounts that they're selling. I suppose that most of these sellers are just scammers.
it can create incentive and when there is incentive they will find a way. it may even create a bigger merit selling market alongside the account selling market! both of which already exist on a smaller scale.
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not letting campaigns paying in altcoins to operate is not only absurd but also it is like erasing the problem instead of solving it. because the problem is not the campaign's payment method, it is in the manager/owner paying people for their spam.
and opposite example is Yobit campaign which was paying in bitcoin and was responsible for a large amount of spam.