After some checks on DB tables of community and profiles where I've found a consistent state, I had check the community event queue like described in this page
The fist step is start a wsadmin prompt ,
than enable community admins evironement
now I've run the command to see the Community event queue (to have shorter command I'll write communityID to represent the community ID and eventID to represent the evenet Id)
CommunitiesQEventService.viewQueuedEventsByResourceId("community", "communityID", None, 100)
this command replied to me with more of 100 events like the following :
Remote App Resource Id Event Type Event Id
Library c060ec74-ddd1-4c62-bdb1-fe0316a760dc widget.removed eventID1
Library c060ec74-ddd1-4c62-bdb1-fe0316a760dc community.members.added eventID2
Library c060ec74-ddd1-4c62-bdb1-fe0316a760dc community.members.added eventID3
Library c060ec74-ddd1-4c62-bdb1-fe0316a760dc community.members.removed eventID4
Library c060ec74-ddd1-4c62-bdb1-fe0316a760dc community.members.removed eventID5
Library c060ec74-ddd1-4c62-bdb1-fe0316a760dc community.members.added eventID6
when I saw "community.members.removed " event I've understood to be on the right way.
First I tried to reprocess the list with this command
but after a check I saw only few event was processed correctly so I moved my research on Community's SystemOut.log where I saw an error while server was trying to remove a widget from that community.
After a double check on the community that widget wasn't there anymore, so I decided to try to delete that event with a dedicated command
the command runs successfully so I tried again to clear the queue with the initial command
and after some minutes the queue was completely cleared.
After this steps if any of the users involved will receive again the mail notification , the last step to definitely fix this issue will be re add and remove the user again from the community.