After a Cheating Player joins a party, he still shows up as a cheater in the scanner, but is unable to be warned/banned or even selected. The selection only shows the party.
You can select an individual player within a party by double-clicking on them. But you’re right that you can’t warn them right now; I need to figure out a way to revamp the UI to make it work!
edit: I think I’ve got a plan to get it working! I’ve set up a task to fix this over here: