IGM pay declined

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I made a $500 deposit on Party for the last bonus through IGM pay. When they attempted to withdraw the funds from my account, my bank would not release the money due to a "possible ACH rules violation". [img]/images/graemlins/confused.gif[/img] It was not a big deal, as I had already cleared the bonus, and was up at the time so party simply removed the $500 from my account. I am curious though, what would have happened if I was down at the time? If I owed them money what could they do other than threaten to ban me from the site if I refused to pay them the difference?