user field modified only by code?

I tried to block a user field in order to be modified only by code.

To do this, my code sets the Enabled property of the user field to true, try to modify the value and sets Enabled to False.

But when I try to enable de userfield, appears an error that tells me that I can't enable user field due to the field is not a item created by de DI...

The only things that I can Enable/Disable are items created by code???