User is able to make changes in Invoice once it is transferred to accounting

Hi experts,

invoice has been created against PI, however still the user is able to make the changes in va02 after the document has been passed to the accounting. Also they have used the Userexits - in include MV45AFZZ, so that the user should not make any changes in va02 once the billing is passed to accounting.

Quick help needed.

Regards

Abhi