My temptation would be to put an A4 adhesive label admonishing the car owner - right in the middle of the driver side - and charge £100 per cup of warm water which they would need to clean it off - after maybe 5 or 6 cups they may learn the error of their ways, or alternatively they could risk driving home with their view obscured!!

This is only a temptation - I am not advocating actually doing it!