[Retr0id]

A more minimalist approach might be a small "x" in the center of each square.

[ramly]

That's a good point. Experimented with a few "big" X's before and it just wasn't doing it. The small x in the center might actually be it.

[ekojon]

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.

[ramly]

That's a great idea too. Thanks guys, please keep the suggestions coming if you have other thoughts.