Email staff, subject "You're forcing down the value of coins" Depending on the value of the transfer and whether or not it's early enough where can call their bank to halt the exchange, it may be a viable option. May incur fees.

You may be able to block it on your bank's side. That may incur fees and the wrath of a support staff wondering what the Hell happened... may also get funds stuck in limbo for a bit, so I wouldn't explicitly advise it.