Anyone know how to deal with a backdoor?

So this guy would often come on my server. He would always be a little bit mingey, but never went as far as to install a backdoor, pretty much taking over the server. I am not sure how to fix this, and cannot find it anywhere. Here are two videos of his destruction:
https://www.dropbox.com/s/2hb259t7ygjkmr...d.mp4?dl=0
https://www.dropbox.com/s/hnxctlq4k9ukjcn/2017-11-24%2021-13-08.mp4?dl=0
May someone please help me with this issue? Here is the guy's steam ID:STEAM:0:0:55572892

One thing he tried to do from console is unban himself, forcing me to say "!unbanSTEAM:0:0:55572892"