If this is a repost, sorry, I searched and found nothing.
If the victim does not have the "Block Trades" option selected in their settings, then you can close their bank/guild bank window by trading with them. Might work with out things, but I havn't tested with anything else though.
-According to XpLoDWilD it will also work with quest windows.
-According to Frito, it will stop you from fishing.
Somewhat common sense, but is hilarious if the person does not know how to block trades.