People will spam to get the minium required posts. If there is pay per post people wouldn't spam in order to actuallly get any type of payment.
In my opinion, pay per post campaign will get more spammers than fixed campaign. But if your main idea is just to avoid spammers, it is simple thing to do. Just kick them out or give requirements to avoid spammers, maybe something like "no one day posting".
But that's already the beauty of this campaign as this campaign doesn't have any users doing "one day posting" and those who are doing are members who don't get time and who are asking for monthly fixed rates...
But, instead of going for fixed, I would still prefer pay per post, as one may still get paid based on their capacity and not for free...
Let's suppose if Bit-X asks for 200 posts a month for a BTC0.4 payout to Hero+ members, don't you think they will be getting BTC0.002 which will be much more for doing nothing???
I know that Bit-X is already having a good reputation now and they don't need such type of advertising anymore, but I guess we are all here for earning on our quality posts and I don't have any shame on saying that...