Excel 2007 : Unlocking cells only when the fomula returns blank

Hello,

With a shared drive, multiple people using the same template.
On Ref Number column next to Product, VLookup is used to return the value that is ongoingly uploaded.
When the uploading is not catching up with production, we enter Ref Number manually.

Problem is:
VLookup formula tends to get deleted and saved as updated template somehow, as a result of Ref Number having entered manually.

Question:
Is there anyway to lock the cells with formula but allow manual entry ONLY WHEN the formula returns blank/zero ?

Would appreciate help, thanks