lol would be awesome if such a spell exists but no. you took it, you have to suffer the consequences be it the effects of the drug, or the drug test. that's kind of how life works, you reap what you sew.
granted, there are spells to help cleanse yourself of negative energy, and there are foods to cleanse your system, but to get rid of THC faster, i don't know. [mostly because i don't do drugs so i've never needed to check] look into foods that might help, maybe say a chant, but i doubt it would do anything especially since magick is all about learning, so unless you're trying to quit, it most likely wouldn't work. [if there was such a spell]