With rampant deployment of wireless technologies such as WLAN, information leakage is increasingly becoming a threat for its serious adopters such as enterprises. Research on antidotes has been mainly focused on logical measures such as authentication protocols and secure channels, but an inside collaborator can readily circumvent such defenses and wire-lessly divert the classified information to a conniver outside. In this paper, we propose a novel approach to the problem that forges a walled wireless coverage, a secure Wi-Fi zone in particular. Inspired by the fact that jamming as an attack is inherently difficult to defeat, we turn the table and use it as a defensive weapon to fend off the covert illegal access from outside. To validate the proposed approach, we conduct extensive outdoor experiments with the IEEE 802.11g Wi-Fi adapters. The measurements show that the forged secure zones match well with the model prediction and that the defensive jamming approach can indeed be used to protect wireless networks against information leakage. Lastly, we propose the algorithms to configure defensive jammers in arbitrary geometry.