As the sender is from @gmail.com Below are the suggestion you can make to resolve spam mail issue.
1.All the recipient must mark the mails are spam,Refer the link
to mark the mails as spam in Gmail.By doing so any mail from the sender will be automatically sent to spam by Google
2.Create an inbound rule to block the sender <email@example.com
> from your admin console as per the attachment below
Note:Replace firstname.lastname@example.org with your sender email <email@example.com>