He is not a registered players, so it is not possible to ban him now.
Of course it's possible to ban him. xD
Go to the Server Chat log, and then press F3 And look when his name entered to the server.
Afterwards, you will see the IP that he entered with him to the server, and then you can ban his IP with /rcon banip [IP] on the server when you are logged as rcon, or from the Console, using banip [IP]
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