This thing will explode. Get ready to monetize or shut it down :) Easy thing you could do is rate limit transfers by default and offer higher bandwidth with a small bitcoin payment.
Please dont go down this route it leaves bad taste.
Any site which segregates its users, oppreses those non-paying, poor people, isnt attractive at least in my book.
An alternative is to do like reddit, offer pay-for-services in exchange for something more, not take away functionality from those poor users to force some kind of payments.
