Hello,
The form personalization is great feature from MS for users. What's the business scenario where you want to restrict them, it wont override any of the business process and logic on form, Also, in any condition a system admin will have full access why you want to restrict even system admin user? its simply not achievable.
In short: D365FO does not provide a supported mechanism to lock the position of individual grid columns or disable column drag drop reordering. Column ordering is a user personalization feature of the platform grid control. Any attempt to enforce fixed column positions would require a custom UI control and is I would not recommended that.