How to block the sale order ?

Hi Experts,

I am doing enhancement in VA01,VA02,VA03.

Normally, sales order will get block based on delivery and billing block indicator in sales order header. Means in status tab, it shows 'over all block' as 'Not blocked' or 'Blocked'.

Similarly, I have added a new list box called 'price/tax block' with 4 values and is updating in VBAK. If no value selected then, in status tab, it should show the 'Over all block' as 'Blocked' and should block the sale order.

Any user exit available for changing the status tab information ? and How to block the total sale order ?

Please Help me urgenly on this.

Thanks

Parinitha