如何识别没有cookie的以前的用户/垃圾邮件发送者？

I am an admin on a chatting website and we get a lot of abusers everyday, disrupting regular chat flow. We keep banning their IPs and they keep changing them with IP-changer software and proxies. Additionally, they always enter using "Private Browsing Sessions" (this is our deduction, because cookie-marking them doesn't work).I am an admin on a chatting website and we get