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If you have any doubt :
- /whois cn to get the player's ip. If you're not admin you won't get the full ip but take it anyway, at least a range ban is possible. Use screenshot for example.
- Take demo : it's absolutely necessary for a blacklist report on official forum
- Note the server with the date and time, this way the server owner can check the server logs
You can get ip from demo in 1.2 version, date and time are in screenshots and demos name.
*Edited by Todesgurke