I've never dealt with anything like this, but I certainly know the pain of getting my level grinding interrupted in a game where leveling is incredibly slow and painful.
Levels only matter in the mode "free roam" in Red Dead Redemption; coincidentally, Free Roam is the best place to level grind. Free Roam is sandbox gameplay basically; do whatever you want whenever you want, and fighting other players in Free Roam is 100% optional.
So of course, I'm level grinding in the best area you can in free roam, minding my own business. Since this area is a versus CPU area, auto target against other real players is disabled. Nobody is around for roughly two hours of grinding; and all of a sudden, I get sniped from some dick aiming to troll my grinding session. Didn't go for a single CPU, the bastard would just wait for me to respawn and try to snipe me.
Of course, 15~ minutes of my counter sniping skills later (which are only average, really... sniping is lame and boring in RDR) nets me about 50 kills on the troll and he got only another two or so, and the fool finally runs off cursing up a rage-storm on his mic. Trolling trolls is always fun. x)