It usually embarrasses the employees but I've been known to yell
"Hey Assh*le, get your *****ng truck out of my way"
One guy protested. I told him, "Look, in 15 seconds this cart of lumber will be where the side of your G*d D*mn truck is parked, your choice whether the truck is still there or not" He moved, then apologized.
It helps to be 6'5", 300#, and have a voice that can be heard for blocks.
The really strange thing is, once you have the cart loaded, it really is easier to just push the cart into the lot to load the truck. The only reason to be under the canopy is if the employees are bringing something out on the fork lift.