Ive never gotten xmpp spam before and now i have. I have very little time and someone (well several account) have spammed me. In response i added XEP-0191 server side blocking to Monal. There are new iOS and mac betas today. It’s barebones, go to contact details and select block or unblock sender at the bottom.
If you run your own server you might be interested in https://xmpp.org/extensions/xep-0377.html , https://github.com/JabberSPAM/jabber-spam-fighting-manifesto and https://github.com/JabberSPAM/blacklist .