Making the Record Read Only for the Previous Owner

Hi,

I have got a requirement such that I have to make the Activities Record Editable for the Previous Owner.

I have captured the Previous Owner Name. I tried creating a couple of Calculated Fields with the below calculation for providing in the USER Property, BC Read Only Field.

Field Name                                   Calculated Value

BRCRM Owner Flag                 IIf ([Primary Owned By]=LoginName(),"N","Y")

BRCRM Prev Owner Flag         IIf ([Prev Owner]=LoginName(),"N","Y")

BC User Property

Field Name                              Field Value

BC Read Only Field               [BRCRM Prev Owner Flag] OR [BRCRM Prev Owner Flag]

From the above, I hope that only Primary Owner and Previous Owner can able to edit the record, for others it will be a READ only. But, it didn't happen.