There are two ways to report a player:
Click on Who's Online and then find the offender and click on their row in the table. This should then allow you to report them.
From the KillFeed select either Last 24 hours or Kills Involving Me. Then you can find the offender and click on their name to report. Make sure you select the category of abuse (glitching, bambi-klilling, etc) and provide a link or description of the evidence.
Admins will review all reported users and take appropriate action as necessary.