Spamassassin is a memory killer. OTOH, it only comes to action when an email is received (as per default, things may be different when you tweaked your installation). You have 512K for 90 users, which is rather low. How much messages do you get per hour on average?
When no mail is being received, there should not be any spamassassin process. Might be worth to investigate why there are so many processes left. You also have 10 zombies ("dead" processes). Could those be those spamassassin ones?
You might try to get rid of these by stopping starting ISPConfig, but I'm afraid you'll have to reboot the server. Watch the swap space being used. As soon as that happens your system will slow down significantly.