restriction to change payment block in F110

Dear Sir/Madam,

Our Accounts Paybale person put payment block "B" in invoice to restrict the payment of the bill. when our finance person

make the payment agaisnt that document through f110, he get error in payment praposal. But through edit praposal he remove payment block "b" and make payment.

How to restrict Finance person to remove payment block "B" in edit payment praposal. we don't want to make this field grey becausue we want to allow user to remove other payment block like "R".

Thanks in advance for your solution,

Regards,

rajesh