Yes - remove the user's access to the arrow  keys - preferably at about the
elbow. :-)

Arrow keys  have specific uses within text areas (i.e. they move the text
cursor). It doesn't surprise me that they don't work  properly for
navigation between fields where some of the fields are text fields.

I doubt that it's practicable to write code to trap the wrong movement and
compensate for it. Retrain the user.

I don't have any user forms  at hand to try this out for myself. However, I
did just try one of my ASP intranet pages that has text fields in it. As I
had anticipated, the arrow keys can't be used for navigation there, and user
forms can be considered similar from a user-interface point of view.