[cabmandone]

Big rocks as others have mentioned... or a sign that reads Hey dumbasses don't drive in my yard
Your call. I'd go with the sign.
One thought on widening. I'd be willing to bet if you widen it, you'll still have people driving in the grass. You'd be amazed at how people with a trailer or bigger delivery trucks watch their mirror when they know there's a couple boulders on their blind side.