Our Server was hacked!

Hello,
I've a problem with my Teeworlds Server! My Server is running under Linux on a website! It is working rly good but 5 minutes ago my server was hacked! If I connect to the console (so if  type my password), I fell out of the server with the message: too weak connection (out of buffer)
If I am only in the server and if I dont login in the rcon, nothing happen! But if I login I fell out!
I only see in F2 and in the console:

This is every time!
[Chat-Command]: 0 used /a
[Console]: No such command: a.
[Chat-Command]: 0 used /a
[Console]: No such command: a.
[Chat-Command]: 0 used /a
[Console]: No such command: a.
[Chat-Command]: 0 used /a
[Console]: No such command: a.
[Chat-Command]: 0 used /a
[Console]: No such command: a.
[Chat-Command]: 0 used /a
[Console]: No such command: a.
[Chat-Command]: 0 used /a
[Console]: No such command: a.
[Chat-Command]: 0 used /a
[Console]: No such command: a.
[Chat-Command]: 0 used /a
[Console]: No such command: a.
[Chat-Command]: 0 used /a
[Console]: No such command: a.
[Chat-Command]: 0 used /a
[Console]: No such command: a.
[Chat-Command]: 0 used /a
[Console]: No such command: a.
[Chat-Command]: 0 used /a
[Console]: No such command: a.
[Chat-Command]: 0 used /a
[Console]: No such command: a.

But the message is there if i am alone on server! How can i ban the IP from him. resp. how can I get the IP from him
Thanks csskevin