How to kill a spammer?

Question

Fight against spam

Spam emails are coming.
Apparently they come in response to a legitimate mailing list.
It's hard to catch him, since the sender changes and ip and helo are ours

Received-SPF: softfail (mxpdd15.i.mail.ru: transitioning domain of s1.frs23.ru does not designate 95.46.8.26 as permitted sender) client-ip=95.46.8.26; [email protected]; helo=mos-ttprint.ru;
Received: from mos-ttprint.ru ([95.46.8.26]:58138)

this is us mos-ttprint.ru 95.46.8.26
this is a spammer s1.frs23.ru
To begin with, I would like to figure out what kind of letters, how he spams, we do not see hacking.