Excel allows you to apply validations to individual cells in your spreadsheet. If you set up a spreadsheet and apply these validations then you will be provided with an error message when invalid data is entered into that cell.To access the option to apply validation first select the cells you want to apply a given validation to. Next go to the data tab and then select Data Validation.
You then select Data Validation a second time and the data validation dialog will open. The Settings tab then allows you to set a variety of different validations for the selected cells.
The Input Message tab allows you to define a message to be displayed explaining the restrictions that apply to the field and the Error Alert tab allows you to specify the error message that is to be displayed if what is entered doesn't match the restriction that you are applying.
Note that the above images have been taken from Excel 2010. The exact location of the option and the appearance of the controls may be slightly different in other versions of Excel. Note also that these steps do not stop the person filling out the spreadsheet from removing the restriction - you would need to lock the spreadsheet in order to do that.