Firstly, you will want to have the command tf_scoreboard_mouse_mode set to one. Once you have done this when you press tab you can hover over the suspected cheater and right clink their name. This will bring up a second menu that will list a few options. Go and hover over report and click cheating.

Now that you have reported them for cheating and VAC will 100% ban them you will want to do the same again but click on profile not report. If their profile or comment permissions aren't private you can write a comment to let others know they're a dirty cheater.

For the final step, now listen closely because this is where is get hard, make sure that you have the console enabled then type quit and hit enter.

That's it and all you can do.