Dear Jaime S,
You can do this by restricting in authorization SHDO and also by marking "changes not possible after release" in Release strategy procedure.
And also you can navigate the menu to, SPRO------>IMG------>Material Management--->Purchasing(OLME)------->Purchase Order---->Define screen Layouts at Document Level---->And go to ME22n And Select the right parameter and in this you can make it display, optional or required entry for the fields.
Regards,
Manjunath B L