I don't have the spell, but if you look into the Vampyre path of Luciferian (Some call it Satanism) witchcraft, you should be able to find an energy draining spell. I have heard it works quite well, but never needed to use it.
the three fold law applies to wiccans and, to a certain extend, to those who believe in karmic laws, not to everyone.
To weaken an enemy might be a good idea to start from not let her/him have too much power over you, I litterally mean don't give that person too much importance, or let that person occupy your thoughts, keep on living your life, unless there are serious matters involved and then you should resort to an authory (techers, police, just as an example, I don't know your story, or how old you are).
You might want to look up for a freezer spell or a vinegar jar to get rid of this person. just type on the search bar.