I'm not sure.
Not that the following is probably gonna help at all, but if 'individual blocking' isn't possible, I guess that just leaves one with the old internet method of simply not opening & deleting unsolicited/unwelcomed/whatever pms or pms from an unknown sender to the recipient.
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" Those who can - do. Those who can't do - teach. Those who can't do or teach - administrate."
- Anon.
" One that will not reason is a bigot. One that cannot reason is an ignoramus. One that dares not reason is a slave." - Anon