Hi Fernando,
If you want some users to have only 1 payment term if they modify the document, you could do it with a substitution.
If you want a user to create a document and another set of users can't change Payment Terms, then I would suggest a BTE 1120. Some more info on http://scn.sap.com/thread/1857003 . In this scenario you will need ABAP code, there is no customizing to change FB02 dynpro for a group users
Kind Regards