I'm assuming when you say they were breaking one of the global rules, you are referring to the "Don't be a douchebag" rule. Generally that rule applies to players who are making the entire server uncomfortable, such as intentionally using racial slurs to provoke others or just in general being a huge annoyance. Going around hitting another player in the
trading server does not really fall under this. There are plenty of ways to get around another player with godmode, the most effective being joining their team. There's also using pyro to airblast them, or going spy and using deadringer or another watch. Or perhaps sticky/rocket jump away. Regardless of what you do, you have to remember this is the trading server. There's really no winning/losing, nor much way to actually grief other players, as it is a server meant for trading.