Setting a calendar field to disabled

Hello

I have a date field called "followupby".
Once the user selects a date then saves the form, I need the field
to be disabled so the user can no longer select another date.

I have set the code in the onsave event to:
crmForm.all.new_textfield.disabled = true;

I have also tried:
crmForm.all.new_textfield.readOnly = true;

but when I publish and test, the calendar button is still available to user.
What have I done wrong?