You might have to specifySelectiveBlkgGate -cell * -inst * first to override any defaults then repetitive unspecifySelectiveBlkgGate -inst <inst> if wildcards do not apply.
Honestly I have never personally used this command , because I am currently stuck in edi v10 because of vendor 65 nm flow requirements.
I still strongly suggest you tell placeDesign where to go, rather than where not to go.
[-cell cellName ]
[-inst instName ]
Specifies cells and instances that are forbidden to be placed inside the soft blockage during legalization.
This command works when setPlaceMode -selectiveBlockage is set to true.
Specifies the cell type name as illegal for soft blockage. You can use wildcards (* or ?) with this parameter.
Specifies the instance name as illegal for soft blockage. You can use wildcards (* or ?) with this parameter.
The following command specifies that cell NR2RLX1 and all instance names that begin with MEMCTLU cannot be moved into soft blockage during legalization.
unspecifySelectiveBlkgGate -cell NR2RLX1 -inst MEMCTLU*