I was playing Grand Theft Auto Online on Xbox 360, and a player teleported to me.
I shot them, but they placed a campfire on me.
Now, I am constantly being burnt to death. How do I get out?
Alright, this is just a classic case of a D-Bag modder. If he keeps killing you and you can't do anything, I guess you'll have to unplug your Ethernet or wifi. If you are willing to take the loading screens again force-closing the software will work. On PC you can Alt-F4 and a prompt to close the game will appear, safely closing it.
This is just a classic modder ruining the game for everyone else. It happens all the time in the older versions of the game, because Rockstar has almost completely abandoned all support for those versions. To get the campfire to go away, you have to completely leave online and reload into it (though it's generally better to completely close the game and open it again, to prevent some bugs and such). Simply switching sessions will not work.
There's really not much you can do about the player themself, because, again, Rockstar has almost completely abandoned the older versions. You can submit a support ticket on Rockstar's website, as long as you know the exact username of the player, but Rockstar very rarely responds to support tickets, especially on xbox 360/ps3 editions of the game. And even if they do respond, it is very easy to just log back into GTAO after being banned, because the modder has to have a cracked system and a cracked version of the game in order to access mod menus in online, and it is a very simple matter to evade a ban in this case.