well a real easy way is if you get a black candle and cut about 1/2 a inch off the top so your left with a little stub of wax and wick, charge the candle (like you should do before any spell) and if you know who has cast the spell concentrate on them and say something as simple as "i ask the gods and goddess to remove these energies, remove the effects of the spells of my enemies, all elements join and help me, rid the negativity so mote it be.". visualize the spell being reversed or sent back to the caster.
that is just something simple you can change it by adding to it if you feel necessary.
hope i helped
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try smudging to remove the left over energies and smells aswell.
glad i helped
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