If you want to send a warning to user.
first go to the member profile(Member who that need to be warned) Or just click the "warning point" as your screen shows.
Then you can warn member.
And i think you are talking about Reputation system.
You can select reputation system
ACP > System Settings > Members > Reputation System > Then select "Reputation type" (Both Positive & Negative Reputation as Webflake)