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To my knowledge, I hope if this is a Wiccan/Pagan spell you are looking for that you know of the Wiccan Rite. Usually it isn't so wise to use spells like this on someone because of Karma and the Three/Ten fold rule. Whatever you send, negative or positive energy, is sent back to you. Also, if the person is stronger than you, they might reflect the spell and you might end up unknowingly casting it on yourself instead of the intended target. Just some food for thought for you. ^^

Since everyone else has already leapt on you to divulge the circumstances, I'll up and say that I don't know a bad luck spell off hand. It'd be fairly easy to write, though, using symbols and elements you associate with bad luck in order to rip whomever a new one, one grazed knee or guano splatter at a time.