Perhaps use a worksheet event which checks the cell when it is entered and
"reports" on any errors.

"TinaF" wrote:

I want to do either "conditional formatting" or "data validation", I
think, so that users will not be able to enter special characters in a
particular cell (/ * \ @, etc).

There is a macro that saves the spreadsheet using the data in this
cell as part of the file name. If they use certain special characters
(such as slashes) it causes an error when they try and run the "file
save" macro.

However, I don't see how to bar the entering of specific characters
using conditional formatting or data validation. How would I do this?