Ma-Ma has this thing attached to herself which will detonate the bombs if her heartbeat stops. Well she does die in the end and yet the bombs don't go off. Did I miss something here?
Dredd reasons that the detonator's signal will not reach the explosives from the ground floor, so he forces Ma-Ma to inhale Slo-Mo and throws her down the atrium to her death. (Source: Wikipedia)
So its clear that building doesn't blow up because of signal range.