this dude comes sometimes to my server and he try to make exact copy of someone else nickname,
not that he says he is the person behind the original nickname user,
but he fooled me and I am angry now
If someone is constantly changing the nick just to annoy or insult others, but does not write silly things in the chat, you can't really do anything else to ask this player to stop it and to find an own and unique nickname.
But if that user also constantly trolls around or even insult others, then please report.