How to complain about a provider? If garbage traffic comes from him, and not from his clients?

Question

Computer networks

Previously, this provider was connected to our SMS gateway, but the contract ended, we blocked their IP, but requests to open a connection continue to pour in.
I wanted to complain to the office that distributes IP blocks, but it didn’t work right away. The RIPE.NET database has contacts for complaints that the provider stupidly ignores.
Where can I complain about the provider itself, for garbage traffic, I'm just annoyed by the constantly growing counters of REJECT packets in iptables
If someone spams me, I can contact his provider (DC, hosting) and he will disconnect this bad guy from the Internet, and if he bad ISP and spamming, who to complain to? Trunk workers? RKN?