You are NOT sending an airplane to a different airport messing with the ILS. Making the large assumption you can send a false signal without the receiver on the other end just popping up an errror flag from 2 competing signals, you can make the plane go too far left - right - up -down in relation to the desired path to the runway and thus make it miss the runway and perhaps crash. Depending on what equipment was on the airplane, it would be more or less obvious to the pilot. If you are backing up the ILS with a GPS derrived glideslope it would be fairly obvious. You would have to be fairly close to the airport to do this. At least one airplane was wrecked with way in WW II by the Germans sneaking a transmitter near an airfield.