I guess they could severely rate limit based on previously undisputed transactions. If it's a new account or an account with 5 undisputed payments over the last

I guess they could severely rate limit based on previously undisputed transactions. If it's a new account or an account with 5 undisputed payments over the last year, it should not be possible to send out 10.000 bills. They could also charge an increasing deposit 10 cents for the first 10 bills, 20 for the next 10 or an increasing percentage of the amount billed. They can refund it if the bill has been paid and not disputed. Should not deter any real user, but gets expensive for someone hoping for this 0.001% success rate.