Oh so your one of "those", huh? What's the good of a lawn if you can't walk on it?
I used to be a postman when I was working my way through collage. I can tell you they probably walk across it anyways, just to spite you. Or they "save up" your junk mail and deliver once or twice a week. The best way to prevent a person delivering the junk to your mailbox from walking on your lawn is to put the mailbox at the property line. Saves the delivery person the wear and tear on their legs walking that extra 100 feet or so to your mail box, which you probably have placed in the furthest possible place from the street, right by your back door where you don't have to go outside to get your mail.
When you have 900 points of delivery to make, that extra 100 feet adds up to a lot of miles. Some people enjoy making others miserable though. It's just human nature it seems.