We should all appreciate that Bitstamp will incur no legal obligations stemming from forum posts that they may or may not be reading.
As others have noted, they will have a designated AML officer, email and registered letter is the way to go. Even a support request.
+1
Seem that many still confuse money transmitter licence what US require and what lead to money seize from mtgox located in US.
Bitstamp does not operate or have any bank account in US.
Each country has at own their financial regulation and in fact depend on laws in their country (UK). At present time I thnk ( not sure) that they can operate without mandatory registration to Uk financial institution.
However, that does not mean that anyone with evidence can not write official report/complaint to bitstamp or law enforcemant in UK in which response and further action would depend on presented evidence.
Forum are not a place for such things.