This question was asked by multiple Twitter followers. The short answer is to use the Report form provided by Niantic, but you can do more than that.

Make sure that the player you are reporting violates one of the following:

Moreover, have valid evidence to support your case. Screenshots from multiple trainers, proof of continuous cheating and other users’ reports help a lot! If everything else fails, and your report doesn’t get resolved in a while, reach out to Niantic Help on Twitter to escalate your issue.

Also, we strongly recommend to focus your reports on spoofers and botters, as Niantic is targeting them more severely with bans.