First of all, you'll have to catch him in the act via screenshot which can be taken by pressing F5 ingame.
then you'll have to go to an image uploading site, something like, imageshack.us
and find and upload the screenshot. It should be located in your folder.........
something like Toribash-3.91/screenshots/screenshot-##-##-##-##
Then simply place the picture here or link us the picture for we can do nothing without legitimate evidence.