unwanted data entry

I have a two field table, where the first fields allows a
user to choose from a drop down box a list of categories
(from a table).

Based on their choice of category, the computer runs a
macro (shown below) which gives them a choice of
subcategories to choose based on their first choice. In
both fields I have chosen limit to list, but somehow the
second field is still allowing data entrants to type into
the field. How do I stop this madness?


(SELECT [Diagnoses Categories].DXCategoryID, [Diagnoses
Categories].DiagnosesCategory FROM [Diagnoses
Categories]; )