Sounds like a Faraday cage. As far as I am aware, this should prevent any signal from disturbing the RX. However it will not protect any stray signals from hitting the antenna (fortunately, otherwise you would have no control).

I am a great fan of solving problems rather than masking them so would not try it myself. Something is causing the interference and my guess is it is internal (otherwise the stray signals would still arrive through the antenna). I would be keen to find out where the problem lies.