It's your property and you have a right to say something. If they are co-workers I understand you have to be a little gentle about it. No problem...they also have to be gentle about backing off. I would never walk off without saying something and explaining that it wasn't ok to do that.
If it's a stranger the same goes especially if you plan to keep parking there. You don't want some prick coming back to key your ride. If you're leaving no sense in being nice unless you're a little guy.
