Some of our clients includes Budget VPS Hosting Provider, such providers are usually magnet for spammers, but they even get some really good customers, but these spammers are like they just buy a cheap VPS For a month, spam, spoil IP reputation and then leave.
Such activities causes a lot of problem for genuine long term customers so to keep such spammers away the most simple solution is to simply block SMTP Port server wide and then just allow SMTP Port on client requests after reviewing their case.
Below command should work on all KVM based VPS Node and it can block SMTP Port for all VPS on any KVM VPS Node:
iptables -I FORWARD -o br0 -p tcp –dport 25 -j DROP
I hope it helps