Resolve Persistent Spamming Issue from CentOS VPS

I need an expert to definitively stop a persistent and severe spamming issue originating from my CentOS 7 VPS running CWP7 Pro. This problem has been ongoing for over 1.5 years.

Problem:
My server is actively sending unwanted emails (spam), which bypasses my Brevo configuration. This leads to a very high volume of "Undelivered Mail Returned to Sender" messages (bounces) in my mailboxes, and my VPS's IP is blacklisted.

Key Challenges:
The source of this active spamming needs to be identified and completely eliminated.
The CWP7 Pro environment presents challenges with persistent Postfix mail server configurations (e.g., config overwrites by CWP rebuilds).

What I Need:
A definitive, server-side solution to:
- Identify and completely eliminate the source of all unwanted outbound email originating from my VPS.
- Ensure my Postfix mail server is properly configured to prevent any unauthorized outgoing mail.
- Guarantee the solution is persistent and survives CWP's automated rebuilds.

Skills Required:
- CentOS Linux Administration
- Postfix Mail Server configuration & troubleshooting
- CWP7 Pro (CentOS WebPanel) expertise
- PHP/WordPress security (for identifying compromised scripts/forms)
- Malware cleanup (if active malware is still present)
- Mail server hardening / Anti-spam

Please outline your approach to solving this, focusing on identifying and stopping the active outbound spam source from my server.