I don't know how withdrawals work, but wouldn't the attacker be able to get your funds without access to your bitcoinmax account?
First PM saying "please change my withdrawal address", then PM again a few days later saying "please send 100 BTC".
They'd have to guess how much you had in your account I guess, and going over might tip payb.tc off that something fishy was going on.
Is there something in place to prevent this attack?
This is a very good point.
Personally I would prefer that *any* withdrawal requests would be GPG signed; however it isn't reasonable to assume that payb.tc accomodate the 14,001 different ways people want this handled.
Putting myself in his shoes I am not sure how I would want to handle that where it would provide good security vs easy management. Remember guys he is only getting %0.1 which is not alot.