You should make an F1 in-game or report it on the forums using the following template in the report abuse section:
Name - (What is the accused person's name?)
Steam-ID - (What is the accused person's Steam-ID?)
Evidence of Steam-ID - (Screenshot or video of the accused players Steam-ID)
Server - (What server was this on?)
Time and date (CET) - (When did it happend?)
Other comments - (Anything else you want to add?)
Reason - (What rules did the person break?)
Story - (What happened?)
Proof - (Any proof (video or screenshot) that clearly shows what happened.)