When I say heading I don't mean a field within the form/template.
I have a standard invoice. I want the payment terms to be fixed but I want an agent to be able to put his/her name at the top of the document using whatever font they choose.
Then, the agent will fill in things like, amount, client name, etc. into specific fields, while other field labels will be fixed (unchangeable).</span>
Tools --> Protect Document
Gives you a few protection options. You allow only form elements to be filled in or make the whole document ready only. Help file on word might point you in the right direction.