Lock a site in sitecollectionusing powershell.

Hi,

I tried to lock a Individual site in site collection using  power shell ,but the the command is locking entire site collection.

The following  command is  to lock a sub site in share point,  In the similar way I tried it for site,but its not working.

Command:

Set-SPSiteAdministration -Identity "http:// Sitename / subsite" -LockState "ReadOnly"

Please share your ideas.

Thanks

Raja