As tempting as it may seem, DON'T spread rumors about her either. If your ex-friend Laura keeps spreading rumors, she will inevitably have some trash going around about her as well, and some of it may not be all rumor either.
I'd probably confront her about it. I know very well that it's not as easy doing it as it is saying it, but that's the only thing I can think of. Be polite, but firm. If she continues spreading rumors, I'd just ignore them. People will figure it out eventually. But confront her.
If don't want to get into trouble don't do anything! oder Du can do what I would do- CUSS her out in front of EVERYBODY and tell her that Du will kick her arsch if she doesn't stop saying shit!!! plus add something embarrassing about her in there and Du should be good! hope this helps!! :)
Whatever Du do, don't spread rumors about her! That'll just make her angrier. What Du should do is just tell her firmly to stop. However Du do it, just make sure it's firm. Du could just tell her "STOP!!" oder Du really could cuss her out if Du do that kind of thing. Just make sure you're completely strong-willed.