Office 365 Email Cleanup

My Office 365 tenant was compromised and spam is both being sent from and funneled into several mailboxes. I still have full access to every account and I have already changed all user passwords, but the unwanted messages keep coming, so the breach clearly runs deeper than simple credentials... (Budget: $10 - $30 USD, Jobs: Active Directory, DNS, Microsoft 365, Microsoft Azure, Microsoft Exchange, Office 365, Security Auditing, Windows Server)