I've got an idea:
There should be a faster alternative way of becoming a member (for those who do not want to spend 4 hours and write 5 posts).
For example, a newbie could donate 0.01 btc and automatically become a member.
Or he/she could make a deposit of 0.1 btc (which would automatically returned to her/him after she/he publishes N useful posts and spends M hours online).
I don't think trolls are ready to pay/risk their money.
Then you're simply making it a pay-to-use website, which is probably not the best for the community. 4 hours and 5 posts is not a huge amount, it really shouldn't take you that long if you actually plan to be active in the future.