Place the sticker on the window of the violating vehicle. The sticker, however, uses an adhesive that can be removed cleanly. The driver is notified and, for the time at least, not un-neccessarily aggravated.
This label is a good first warning.
Parking violation stickers are sold in packs or a book of 50 labels.
No parking ZoneThese stickers work great for cars that park in no parking zones