Banning an IP Range

How would I ban a range off IP’s in-case someone has a VPN or something, someone said i could do this…

Because there’s some hacker with Alt Accounts and a VPN, who keeps joining my server and banning everyone.

How are they banning?
Maybe you should remove their steam id from whatever Admin list panel you run?
Do you have sv_download/upload enabled?

sv_download and upload are both on 0.