BADI or EXIT to block parking of document

Hi,

I have a requirement that after GR,i have to check in whether the quality chech has been done or not.For that i have to check whether the LMENGEZUB field in QALS table is empty or not. If it is empty then i should be able to park the document thru transaction MIRO.If the field is not empty then I have to block the user to park the document.Can anyone please provide me a solution as how to achieve this or please provide me an exit name or BADI name to do it. I have tried using the userexit LMR1M001,but was not successful.

Please help....