You will also need to gather some proof of him doing this. The Staff of ZARP can't just banned someone because you said they are griefing.

I will list some ways on how to get the proof.

Notes -
The console starts disabled by default. To enable it, load the game and visit Options > Keyboard > Advanced. There will be a pop-up that will include two options, one of them labeled "Enable Developer Console".

1st method (Screenshots)
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To achieve this you must press F12 and it will take a screenshot and be saved for later usage.
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2nd method(Record Demo)
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To achieve this you must open the console by pressing "~" then you will be presented with the console
once inside the console you can type record <name> and it will start recording. you can make sure its working by looking at the top right of your screen. should say REC in red letters. Once you have recorded enough game play reopen the console and type stop to finish recording.
Location of the saved file > Steam\SteamApps\common\GarrysMod\garrysmod\
Look for <name>.dem
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3rd method (Record demo using gm_video)
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Why is this method different from record, well the gm_video record everything like the 2nd method but saves it in the format .webm making it so you can upload right to YouTube hassle free.
To achieve this method you need to open the console using the "~" once the console is open you simply type gm_video. record till you think you have enough proof. then simply type gm_video again to stop it.
Location of the saved file > Steam\SteamApps\common\GarrysMod\garrysmod\videos.
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4th and final method(using a screen recording program)
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You can use a screen recording program to record everything if you choose to not use the above said methods they are simple and easy to use I will list some of the ones I know. you can check them out, keep in mind some might not be free but will have a trial version.

Open Broadcasting Software (Free)
Action!(Cost Money)(Trial Available)
Fraps(Cost Money)(Trial Available)
CamStudio(Free)
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Good luck on getting the annoying griefer banned!

-Tea Leaf-