Sticking leaflets under a car windscreen wiper, could constitute tampering with a motor vehicle, which is a Criminal offence. Although, I don't know of any instance where that proposition, has been tested in court.
I know that it's certainly a problem for householders in my area, with flyers for takeaway pizzas and kebabs, local store franchises etc. Most of this detritus, goes straight from the letterbox, to the recycling bin.
Posting the stuff through your letterbox, AFAIK, is perfectly legal, albeit a nuisance.