The first thing to check is the origin of Spam. Unfortunately, this task can be somewhat complicated if you do not have the right skills, because you have to check the tail post, server logs, etc.
The location of the logs depends on whether you have installed Plesk panel on the server. If so it would be /opt/psa/var/log/maillog.
To remove the IP lists, you should check first of all what is ready. You can use a URL like the following to check:
Once you've checked that lists reside, will have to contact / perform the procedures relevant to each for removing the IP. However, it will be useless if not eliminated the source of Spam as they return to list the IP.
I hope I have helped you, if you have more questions about it, let me know :)