In the spec for the ledger format, there's a flag called "lsfDisallowXRP" in the account root entry. It says: "Indicates that this account does not want to receive XRP." But why wouldn't an account want to receive XRP, given that it's necessary for doing any transactions?
This was created specifically to resolve a problem Bitstamp was having. Though there is absolutely no reason to send XRP to Bitstamp, people were constantly doing that. Bitsamp had to either steal the XRP or refund it. Obviously, they wanted to refund it. But this would drain their hot wallet of XRP.
To resolve this, we're in the process of adding the flag you mentioned, "lsfDisallowXRP". If the flag is set, the client will not let you send XRP to the account that has set it. Bitstamp would then set this flag.
Bitstamp can still put XRP in the account to fund its transactions by simply ignoring the flag. We may also add a way to override the flag in the client, possibly by putting a special marker at the end of the address or possibly with a button.