Equally, I take your point Dave that your not really ever perhaps going to stop the bad guys with the solution I suggest....if those types are determined to hack firmware and/or geo-fencing solutions built into drone firmware and software, then I would suggest that given a level of expertise needed to do as such, then they are really hell-bent on creating chaos akin to terrorists - fair comment?.
What I'm suggesting would merely be a solution to your average 'Joe Public' that would buy a consumer drone and then think it would be fun to throw it up to get close-ups of airliners at airports. That at the moment is where I see the problem being - and its a lack of accountability I suspect that makes these types feel they can get away with what currently happens by way of reported incidents.
The day that an individual with some form of desire to cause harm with a device and their ability to remove monitoring/flight restrictions appears, then sadly another method does need to be utilised - and this already exists in the form of opposing forces using shuttlecock bombs (Iraqi soldiers v ISIS). There is significant cost here agreed, but that could be true of anything involving the mitigation against acts of - well, bluntly its terrorism.