Your options here are fairly limited. It doesn't actually sound like either player here was cheating... GTA is a player vs. player game, and people who lose battles tend to be salty about it. From the Rockstar point of view, it would be pretty hard to separate the wheat from the chaff, so to speak, in terms of reports.
One thing you can do is "report", and then "vote to kick." Both of these actions assign bad sport points, and if enough of your friends kick-vote (usually 3-4), the player will be removed from the lobby instantly. You do both of these actions via the "Players" menu under the "Online" tab of the pause menu.
As far as I can tell, players are rarely, if ever, banned for being mean and nasty, or using glitches/cheats. At worst, the game sends them to a bad sport lobby if they accrue enough bad sport points. So, reporting and kick-voting is a mere inconvenience. Instead, I find it much more satisfying to annoy the hell out of the cheater, such that the lobby is no longer fun to grief other players in.
If a player is using a God mode glitch, sit on your orbital cannon in your facility and blast them every time they get on a vehicle. It won't kill them, but it will destroy their vehicle and make it hard for them to do anything (you'll get bad sport points for doing this, though, so have your friends commend you via the player menu).
- Note- there are a couple of glitches where this won't work. One is the god-speedo glitch, which the player is vulnerable but their vehicle is not. You have to shoot them out of the vehicle in this glitch. Another is a god mode glitch where they appear to be in their CEO office. The player cannot be targeted with orbital cannon when the game thinks they are in their office.
If a player is killing you repeatedly, ensure that they destroy your personal vehicle in the process. After a handful of times, they'll get placed into bad sport mode. Do this in combination with reporting and kick-voting to accelerate the process. Once you see them wearing a dunce cap, they are done for - keep away until they are removed from the lobby.
Make some higher-level and/or try-hard friends. Play heists or join MCs in your lobby to help with stand your grounds. I look out for lower-level players and I'll come to the assistance of anyone who asks me for help.
Learn a few glitches on your own, then deploy those against the griefing/cheating player.