The OnObjectEnter Script Trigger can selectively deny access to the field by performing a script that checks to see what user is accessing the field and uses go to field or go to object to move the cursor back out of the field if access is not to be permitted for that user, but drag and drop actions do not trip this trigger, so you'd need to back that up with a validation rule or the OnObjectModify trigger to prevent all possible methods for modifying the field.
An alternative approach is to move that field into it's own table linked to the original table in a one to one relationship. This enables you to use custom access privileges to deny access to the record form this added table and thus prevent access to the field.
Thank you very much,
Went for the second option and it works great.
Using Phil's method to stop access, there is another (serious) method (to use with it). Which is to create a Privilege Set, and set both of these people to use it (with there own Password). In the Records option, at the far right, is Field Access. You then want to choose "Limited". There are 3 options: modifiable, view only, and no access. You want "view only" for almost all of them, then "modifiable" for that one field.You can select them all, set to "view only", then reset the one to "modifiable." It is not that hard to do. You can be very sure that they'll never change the other fields. Be careful however that no scripts you want them to use change anything. You might want to let them access some global fields also (as they generally are safe, and may be useful).
Thank you as well. I will try this option as well.