Lord Vicious, Leviathan is right, a mod in the game can not ban on screenshoot proofs without tribunal case, but the easiest way is to just report it instead of making screenshots, every report is checked. Please, use report function for chat abuse cases as much as possible and leave tribunal for game mechanic issues.
Caesar Augustus, players should listen to moderators, when they warn to keep the chat civil, if a player did not listen to it twice, a 1 day chat ban with a report is a legit action. A player can use a lot of more civil words to asking to stop instead of "stfu" and just report it.