If a user types in data not in a combo box list, how can I write a macro to delete that text and return the control to a null entry>

You can avoid having the user enter information that is not in your combo box. In table design, under the Lookup Tab, set the Field Property "limit to list" to YES. When the user types in an incorrect entry on the form, a box will pop-up informing him/her that the text is not in the list and that s/he must re-enter the field.