Supressing Fields in Sale Order

To make optional / mandatory you can use in IMG - S&D-> Basic > Functions-> Log of incomplete Procedures =>  select the fields > from the tables and the system will check for them (OVA2/VUA2)
To make a filed entry enabled or grey (non-entry allowed):
User exits in the program MV45AFZZ-USEREXIT_FIELD_MODIFICATION
This user exit can be used to modify the attributes of the screen  fields.
To do this, the screen fields are allocated to so-called modification groups 1 - 4 and can be edited together during a modification in ABAP.
If a field has no field name, it cannot be allocated to a group. The usage of the field groups (modification group 1-4) is as follows:
Modification group 1: Automatic modification with transaction MFAW
Modification group 2: It contains 'LOO' for step loop fields
Modification group 3: For modifications which depend on check tables or on other fixed information
Modification group 4: is not used
The FORM routine is called up for every field of a screen. If you require changes to be made, you must make them in this user exit. This FORM routine is called up by the module FELDAUSWAHL.
With Compliments by: Taner Yuksel
Actually suppressing fielding sales orders userwise is quite easy. We are doing it in our company. For this we use userexit FORM USEREXIT_FIELD_MODIFICATION in MV45AFZZ.
Below is the sample code
IF SCREEN-NAME = 'VBKD-ABSSC'.
AUTHORITY-CHECK OBJECT 'ZMV45AFZZ' ID 'SCRFNAME' FIELD SCREEN-NAME.
  IF sy-subrc = 0.
    SCREEN-INPUT = 1.
  else.
    SCREEN-INPUT = 0.
  ENDIF.
endif.
You place the authority check object in authorization profile in the role of the users, who should have access to the field (in this case it is VBKD-ABSSC), and there assign the corresponding fields that are to be accessed via this userexit.