AFAIK, that depends on how evil his/her ip when registering on this forum, more evil the ip, then more higher he/she need to pay.
Not every new member need to pay some bitcoin when registering, only if he register using evil ip which usually if he/she using vpn or tor.
But the evil ip just charge you a little amount of bitcoin and if someone will use a vpn or something like that they can abuse and create more accounts just saying