Column permission being ignored by Access Form Combo box

[kellysc]

I have a table linked to an access mdb which I have set permission on
for a specific set of users to update, delete, etc and another set of
user to only select, but I need these users to be able to change one
other field in the table.

I have set the permissions and all works well, but for some reason one
of the fields that should be select only is updatable.

It appears it's related to a bound control in a form that is being used
as a combo box with a row source populating it from another linked
table.

All in all I can't see why the row source and the combo box should
create this problem, but all other fields work as I would imagine.
Only this field is a problem.