Hi Dolby86,
It might be possible to make a piece of equipment that will detect
the presence of a mobile phone that is switched on.
They transmit their presence to maintain contact with nearby repeaters.
I think its short bursts about three seconds apart, i could be
wrong, but its something like that anyway.
Such a piece of equipment could be used to show when a mobile phone
is present and switched on.
Such a piece of equipment would not necessarily interfere with any
thing else, but if it gave a repeated beep then the owner of the
mobile phone would most likely turn it off, or leave.
This might be a better course of action than trying to 'jam' the signal
which has already been discussed. I would just add that i think the
'spread-spectrum' method was originally devised specifically because
it is almost impossible to 'jam'.
I don't know how similar the mobile phone system is, to the original
design put forward by H.Lamarr & G.Antheil, but that was the intention.
Best regards, John