Google Groups allows spammers to subscribe you to their groups in a workspace account, but doesn’t allow you to unsubscribe since you’re not part of that workspace account… Worse - all autoresponders for the spam will also be forwarded to all members of said group.

Since I do not participate in Google Groups, the nuclear option is to file all mails from Google Groups as spam with a Sieve script.

Note the require part may be too large as may sieve script also contains more rules than this one.

require ["body", "date", "editheader", "envelope", "fileinto", "imap4flags", "mailbox", "regex", "reject", "variables"];

if anyof(
    header :contains "List-Subscribe" "groups.google.com",
    header :matches "X-Google-Group-Id" "*"
   ) {
	if header :matches "Subject" "*" {
		set "subject" "${1}";
	} else {
		set "subject" "";
	}
	deleteheader "Subject";
	addheader :last "Subject" "[Google-Groups-Spam] ${subject}";

	fileinto "Junk";
}