Doing this while the accounts are inactive is hard, but I might have an alternative solution for you here.
You could re-activate the accounts and then manually change their user preferences in the ACP. Go to the Manage Users module there and choose "Preferences" in the drop-down after typing in their username. There, you can set the option "Allow users to send you private messages" to
No.
As a side note, support for phpBB 3.0 has been dropped a while ago, meaning that no more support and security updates for it are provided here. Have you considered upgrading to a more recent version?