Technically you could file a report in-game, as there are multiple very broad subjects for filing a report, but there isn't one specifically for "kill stealing".
As far as getting banned for the offensive, it just depends on the GM handling the case, repeat offensives, etc etc.
Ask them to leave and report them if they don't, it's technically against the rules.
Other than waiting for them to leave or get banned, no there isn't really anything.