Nah, try the universal "don't go here" indicator of close diagonal lines like you see in parking lots. Keep the shading of the lines and the square background relatively similar so they convey the message without distracting from the rest of the board.
Yeah I think that might actually be a good idea to try out. I tested a few things with shapes during the beta but they kept looking too '3D'ish for the rest of the theme. Not sure if I'm explaining it right but the obstacle shapes kept popping out of the background somehow. Do you know what I mean?