I conditionally formatted a range of cells so that a cell changes color if
the content =x. How can I lock the cell if it contains an x?

The scenario is an employee schedule. I put an x in the cell of days they
can't work. But I've deleted the x by mistake and scheduled the employee. I'm
hoping the color change will help me but would like the cell to lock if I put
an x in the cell. Then I'd need to know how to unlock an individual cell if
the employee changed their mind.

I've seen posts about conditional format\custom but can't find it in 2007.
Thanx