@josecgomez 's method is the appropriate way to handle this. If you have not yet done this, read no further!
If for some reason you have already done this and are still having an issue, @Randy 's method should update this throughout the system if you do not want it to be editable unless by exception.
If you are still having issues, you can explicitly assign the property to the control on the form load, but depending on the form you may need to put this multiple places. For example, create an on form load method using the form event wizard:
Then, add something like txtControl.IsEpiReadOnly = true; of course, you’ll want to put in the name of your form controls.