Jamming a signal is not a good idea , it could also block more than just wifi signal ; these devices could disrupt important communications services that operate on adjacent frequencies, or worse, they could disrupt all communications within a broad frequency range .
According to Federal law ,it prohibits the operation, marketing, or sale of any type of jamming equipment, including devices that interfere with cellular and Personal Communication Services (PCS), police radar, Global Positioning Systems (GPS), and wireless networking services (Wi-Fi).
http://www.fcc.gov/encyclopedia/jamme... more info .