[cs_waller]

When you start your server write in the console banip HisIp

EDIT:If you don't have his ip go to server server_log.txt in your server folder and open the file. Click ctrl+f on your keyboard and write in the dialog window attacker's name ( if you remember his name ).Then you will see this

EXAMPLE:

[19:14:24] Incoming connection: 192.168.1.4:4149
[19:14:27] [join] QwerTY has joined the server (0:192.168.1.4)

[HOUR:MINUTES:SECONDS] Incoming connection: IP:PORT
[HOUR:MINUTES:SECONDS] [join] Name has joined the server (slot:IP):

Copy the Ip from txt file and go to console then write banip HisIp