The Captain has final authority over an aircraft’s operation, however in the event of a GENUINE hi jacking or bomb on board the local authorities would be justified in keeping the aircraft away from built up areas or potential targets and a forced diversion to a suitable facility would be in order.
This however, was state terrorism along the lines of a rogue state or 1980s Libya and needs to be dealt with seriously, as any similar action by an outlaw organisation would be. Making false bomb threats and unlawful interference in the operation of an aircraft just to start with.