This is very likely, impossible.
Even if you would attenuate the signal, it would all depend of the sensitivity and power of the other pair.
While you might be able to reduce your signal and on a particular device it would be only visible for a certain distance, it would totally change with another device.
If you do so for security reason, it would only take someone to have a sensitive bluetooth device, or directional antenna to get the signal.
Also RF signal is quite unpredictable, it can be reflected off objects, absorbed or go through.
The only plausible way to have a RF signal locked to a certain area would be to have a Faraday cage, for example covering the walls, windows, floors with a conductive paint, or some sort of conductive shielding.
Alternatively, you could build a sort of small Faraday cage enclosure where you can control the signal, have the transmitter inside at lowest level.
It wouldn't be perfect, in which the person would have to put his device in order to get the signal.
Something like that (don't laugh at the image, you get the point :P).