Email spam randomly started

Randomly Last night EMAIL SPAM from my domain started! I host a website and it's been attacked before in 2017. I had to start fresh. I Have SMF Forum installed and left it.. Never did anything with it, and bots took it over lol. But last night and today I kept getting emails of spam from boards@forums dot cashisonline dot com I went in and changed email then to random new email i made in cpanel to SMF out of curiosity if that was the culprit! For my surprise it was. I put SMF for maintenance mode and it stopped. Was the forum hacked? How do i fix if so.. malware? rootkit? I'm running immunify 360 with cloud Linux also and it isn't find any viruses. Also, how is

/dev/md1

164G

133G

23G

86%

/

only 23 left.. how? I Ran all the clear commands from this website