Try Data|Validation.

You can specify a list that will stop you from typing anything but things on
that list (as well as allowing you to clear the contents of the cell).

See Debra Dalgleish's site:
http://contextures.com/xlDataVal01.html

George wrote:

I have a column where the entries are always going to be one of four or five
codes, for simplicy sake lets say it is A, B, C, D or E. How do I set it up
that if anything else but one of those letters is entered, the cell will not
accept it.

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Dave Peterson