insert State name exactly same as dropdown list

Hi,
I made dropdown list column B6 to last row for state abbreviation like NY, CA , and so on.
But some users use the same letter but using lowercase like ny, ca, and so on. I can't stop them.
Later when I filter data, I will get problems.
How can I force users to use dropdown list?

I used input message in Data validation and error message in Data validation too, but that can't keep users from inserting lowercase letters.

Thanks

Thanks, it worked!