In the U.S. *operating* an unlicensed transmitter is, with a few exceptions, contrary to law. _Operating_ a transmitter, whether licensed or otherwise, to "deliberately interfere" with the operation of another licensed transmitter is contrary to law.
Pure "passive" measures, however, are *NOT* proscribed by law. Among other reasons, because your signal has no 'right' of transit across somebody else's private property.
Engaging in a legally proscribed activity that has the side-effect of "danger to life and limb" of un-involved third parties _does_ open one up to civil suit from those who suffered adverse consequences from the _proscribed_ activity -- as well as the risk of criminal prosecution for those actions.
An aside, I have direct knowledge of one church that actually Faraday caged their sanctuary. In fact, they did it nearly *FORTY* years ago, now. They were undergoing change from 'traditional' to 'modern' style of services, including a complete remodel of the sanctuary -- with a very "open" area for the ministers, etc. No podium, lectern, etc. So, they went with _wireless_ microphones feeding the P.A. system.
All the various changes were perceived to be a -major- improvement, attendance was climbing, more younger people were being drawn in, etc. etc. Then there was a momentous Sunday morning, when somebody with a high-powered mobile radio (probably an illegal CB rig) drove down the street, during the sermon. There was a "most unfortunate" juxtaposition of his language over the minister's sermon, as his transmission overloaded the receiver for the PA system. There wasn't any practical way to eliminate the wireless mic's, while retaining the 'character' of the new-style services, so they _did_ shield the entire sanctuary to prevent any recurrence.