1.Just Login the Particular SAP User with ID & Password.
2 Open the Form on which you have Hide the Fields.
3. Go in Form Seetings - Table Format - Enable/Disable the Visible Check Box.
4.After that Suppose Sometimes User can Access the Form Settings & May change the Visibility of Fields.So Authorize them.Check Below for this-:
5.Go-Administration - System Initialization - Authorization - General Authorization -Select User - Click to General Option -Document Settings in Front of that Select No Authorization.
Please check Attached Image.