Excel

You can use Excel's Data Validation tool to prevent the user from entering anything except text in the desired case. Select the range of cells to which you want to apply Validation, choose Validation from the Data menu and choose Custom from the Allow list. In the formula box, enter any one of the following formulas, depending on what case you want to allow. Change the reference from A1 to the address of the first cell in the selected range.

=EXACT(A1,UPPER(A1))
=EXACT(A1,LOWER(A1))
=EXACT(A1,PROPER(A1))

With this validation in place, the user can enter only upper, lower, or proper case.