No a paintball won't go through 1/2 inch of wood unless it's completely rotten or balsa wood, MDF isn't wood so you asked the wrong question.
If your aim is to stop paintballs from going into neighbours gardens i would suggest an 8x4 foot sheet of board isn't big enough unless you're shooting at it from a couple of feet away, all it takes is 1 barrel break and the next shot could quite easily miss that board and go where you don't want it to. Site use netting to keep paint out of places it's not wanted to go, i'd suggest using that, builders scaffolding netting is ideal for this.