You could remind them one last time that it's a felony, and if they continue, set the law on them. Once a couple are arrested, tried and executed for murder, the others should get the picture.
Okay, that may be a little too far in some respects, but the only real way I can think of to get players to appreciate that they are breaking the law is to get them in trouble with the law. Prison time, non-fatal capital punishment and permanent exile are other options to execution. Or some type of very close call for one final reminder.