thanks guys.

yah, i would like to warn AND aggravate the driver 

Hey, long time no hear! How's the family?

Do plain white labels are definitely a possibility, but the client really wants it difficult to take off. Apparently the offensive vehicle drivers are chronic, regardless how many times they've been told not to park there.

My advice was to take the company forklift and just physically move the offending vehicle. Technically, it's within their right - it's on private property with public access, they can refuse the right for anyone to pass through, much less park there. 

i had the same problem with people parking in my private spot i pay for... i used a 81/2x 11  Avery sticky label, printed a lot of bla bla bla on it then put it on the windshield right where the driver would need to see to drive, you cant get those things off glass and when you  do get a corner up it tares.. and leaves glue like crazy and if it gets wet even messyer, not many people carry a razor scraper with them all the time so when they hop in to go its well ugly. try some you are going to love it. needles to say i only post a label once a year or so now.