Conditional Formatting to Lock a Cell

Squiggler,

I am locking the sheet and they will only have a limited group of cells in which they can enter information... but SOMETIMES, they have to enter data in Column C but only if someone else did not enter information in Column B.

I don't want them to enter in information into Column C if Column B already has information in it... and they will, even if they are told not to, unless I lock them out of it.

VBA seems the best option here... and I'm halfway to a solution in that arena.