A bit of pre-planning helps. If you know it’s going to freeze overnight then turn on the windscreen wipers and when they are at some suitably jaunty angle turn off the ignition. Now put some newspaper over the windscreen using the wipers to hold it in place. Not brilliant if the windscreen is damp at the time though.
Other than that: warm water from the tap is most effective. I like the hot water bottle idea.
Leaving a vehicle unattended with the engine running on a public highway is, if not illegal, then stupid. Dead easy for someone to jump in and drive off. Maybe the OP could do that and just park the car round the corner – at least it would impress on the idiots that they are being idiots.