You're not being particularly clear about the environment around the metal detector or the type of metal detector you're using. Not all metal detectors work in the same way so it would be useful to know exactly what you're using. Perhaps you need to explain it all in further detail.
Anyhow, for now there are two possibilities that you can explore. The first is to attenuate the signal from the metal detector to weaken it. This could decrease its sensitivity over a certain area with the result that it might not detect the metal in the walls and so would not be activated.
The second possibility is for you to line the room from floor to ceiling all the way around with wooden cladding. The cladding would have to be fixed with panel adhesive as would the wooden support bars placed horizontally across the room that you use to secure the cladding to.. No metal must be used at all in the construction. Don't worry too much about the gap between wall and the outer surface of the cladding - the bigger the gap the better. to secure the cladding to. Again you may have to attenuate the metal detector but you should definitely get to a point where no metal is detected by the metal detector.