If you want to report cheaters/hackers/modders, you can do so by submitting a support ticket, sending a report here at GTAOnlineBanhammer@rockstargames.com
or by posting here on the Forums.
When you do please include the following:
-Platform (PS3/PS4/Xbox 360/Xbox ONE/PC)
-Cheater’s Gamertag / PSN ID / Social Club Nickname
-Description of the violation
-Approximate time/date of the violation
-If possible, please include concrete evidence, such as a video or a screenshot