I would suggest charging after a number of accounts register per IP.
If we say that there can be 2 accounts per IP made per month, if anohter account is made (or 2 unmerited accounts are open on that IP) the user of those accounts must buy a copper membership/pay 0.001BTC to create their next account. That would probably work better than limiting everything in a more general sense (it might still increase the number of sales of newbie accounts and brand new accounts though).
I think that will still contribute to the spamming, the key to promote quality posts on the forum is not by account restriction by IP. I think the payment of some amount of bitcoin when a newly created is ban from posting is already in place. One of the ways to prevent users from spamming is by discouraging users from commenting on trash posts. Hence systems such as a way of demeriting members who spam can be implemented.