You can block nuisance players or unblock accidentally-blocked players. Blocking a family will block the family's individual trade, whisper, and squad operations. Unfortunately, it doesn't stop him from spamming duel requests while you are busy checking out the market.
Use Alt-F3 to open up the Block List. You may then add/delete family name directly. (This block list isn't listed under Menu, so you must use the hot-key.) You may also right-click on a character and select Block instead.