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You can do this by recording him with screen-capture software like Fraps, Bandicam, Screencast-O-Matic and other.
Then, upload the recorded sample on Youtube, and send it to the Game violators section on the forum.

Sincerely, NedFighter
Hello, 

You can only use the test server when there is a need for mass-testing. Only official testers have the code for entering the TO test server.

Sincerely, NedFighter
Hello,

Fly hack is a third-party software which enables a tank to fly.  This hack can be very annoying, a player can steal gold boxes with it, capture flags and other.
Some maps were removed due to this hack, like Moon Silence.

If you want to report a fly hacker, record him and send the sample to Game violators section in the forum.