It is a great idea to terminate/kill almost 99% of the spam messages by charging fees, scammers hates to pay, but they love to take other people hard earned money for no return.
This approach is really good IF Fiverr at the highest of the freelance mountain (and they are) + there is no one near to compete, this could solve a huge problem by creating another, I mean who could pay first to just get access to a freelance market without considering alternatives?, And then start spending?. specially if they've never heard of Fiver before.
Some freelance marketplace will rise up and take advantage of the situation by marketing against the core idea of fighting spams, example: Are you paying to just hire a freelancer? Sign Up It's FREE and so on, this could limit Fiverr ability for growth overtime and they know it.
The bright side is most of the seller will work comfortably with trust anticipating a new message as a new lead not spam and they will reconsider leaving the platform 10 times before making a decision.
The question is: Will Fiverr really consider locking the door like an ATM to protect its sellers?
Apologize for the long comment, I'm not hear to argue.