No level. Forget about it all, it's pointless. GTA Online has wanted levels that make you hunted by tanks and helicopters, doesn't do diddly squat, there are tons of players who are good enough at the game to stay at a five star level for hours and kill hundreds of innocent roleplayers, even when they're in passive mode (run over by cars with nobody in them and stuff like that). A killers pride and joy is to beat the system, and unless you make the game unplayable for everyone they will always be one step ahead.
To me the only option ever to solve this problem is to hide the fact, that you're a human being. This automatically drives all the killers, who need the confirmed player kill to enjoy it, out of the game and leaves only the role players who just would like some presumed NPCs to unexpectedly answer their hails.
Unless you are willing to completely remove any and all motivation for killers you are not going to get a significant number of PvEers back into Open.