If a Player has caused you a negative play experience you should start eviting him with the /block feature if he is insisting and not leaving you playing peacefully you should proceed on reporting him in Forums.
Typing /block "Player name" will stop the messages from appearing.
You will then be able to report the player for various negative behaviors:
Harassment: Offensive Language – Including, but not limited to language that is vulgar, obscene, sexually explicit, or racially, ethnically, or otherwise objectionable.
Harassment: Verbal Abuse - Including, but not limited to, language that is unlawful, harmful, threatening, abusive, harassing, defamatory, or otherwise objectionable.
Spamming: Whether for personal or commercial purposes, by disrupting the flow of conversation with repeated messages of a similar nature is a reportable offence.
Botting: If you want to report a player who's using external programs to get advantage over others, remember that reporting a bot needs screenshots or/and videos.
In order to post a player report you should explain everything (how all started, what is he doing, etc), in the tittle should appear "Name and Reason" (example: "Jolan is botting" or "Relix is insulting me" Make sure you give us proof both with videos or screenshots.