If the only way to update a field in a file is programatically, you can add an edit to that program(s).
We all know that files are updated many other ways: CPYF, DBU, UPDDTA, SQL, etc, the only solution is to add a trigger program to the file. In the program you can vaildate your field and then take whatever action you want.
You may want to convert those invalid characters to blanks and sent out an an error message.
Depending on the possible values in the field, you could just add a Constraint ADDPFCST