XMPP servers properly compliant with XEP-0157 expose contact addresses for reporting abuse. At the moment, Gajim must be used for sending queries to obtain those addresses; I don't know of any other clients that expose an XML console at the moment.
Paste the following snippet into the console and replace
email@example.com with your Jabber ID (JID) and
offendingdoma.in with the domain you want to obtain contact info from.
<iq firstname.lastname@example.org' to='offendingdoma.in' id='disco1' type='get'> <query xmlns='http://jabber.org/protocol/disco#info'/> </iq>
Look at the output and search for this snippet:
<field var="abuse-addresses" type="list-multi"> <value>mailto:email@example.com</value> <value>xmpp:firstname.lastname@example.org</value> </field>
The addresses in the
<value> tags are who you should contact.
mailto: indicates an email address and
xmpp: indicates a JID. I recommend just picking one. In either case, contact that address and wait 7 days for an appropriate response before contacting their ISP. If the ISP doesn't take action, open a PR adding that domain to JabberSPAM/blacklist.