. . .I have to understand how my ISP reacts to this kind of things.
For the moment I will keep a low profile and I will block the mentioned IP range for a month.
Webiron's system sends notifications to both the abusix.org contact for the IP and to abuse@base-domain.tld for the reverse-DNS name of the relay IP. So if you can configure abuse@ for the relay domain to forward to you, you will see their notices at the same time as the ISP abuse desk. Might be helpful to know about it before they contact you and/or to see if they become familiar enough with the notices to ignore them. Automated abuse complaints from other sources do not always go to the domain-based address.
is a handy resource that shows the abuseix.org and abuse.net information, as well as how many DNSBLs the relay has racked up. You can change the abuse.net contact but Webiron appears to ignore this source and simply construct the abuse@ from the rDNS domain name.