Duplicate check numbers

We are running on 9.2 and recently had an issue with the duplication of check numbers.  We all know this is primarily because the user changed NXTC during the writing process.  I am looking for recommendations about how to avoid this issue, other than asking the user not to override the value coming from F0030.  The business wants to set security on the field, which as far as know would be a customization on the application directly, but wonder if this change would have other negative impact on the business that we are not able to forecast now.

Your advise is really appreciated.