How to stop Trello email notifications from a Trelloboard where I'm not added?

[TatianavanB]

For my team named Project Z, I have three Singleboard members named Mascha A. Pim A and Richard B. They are getting notifications from the Trello board named "AR", but they are not added in this board.
How is this possible? And how can it be resolved that these persons don't get the notifications.
They are receiving the notifications in the personal inbox and in the group inbox from the Outlook email. 

Greetings, 
Tatiana

[Blair at Atlassian]

Hi @TatianavanB

I sent you an email from support@trell.com with a few questions to help us gather the necessary info to inspect those unexpected notifications. 

Thanks so much!

[Iain Dooley]

@TatianavanB this is the second report like this I've seen recently I've asked the atlassian team if there is some change that may have resulted in this 

[TatianavanB]

@Iain Dooley  Thank you. 

I look forward to hear from you.