Can you drop your current spell without using it?
Solution 1:
As far as I know there is no way to drop spells without using them. The best alternative would be to just quickly use up your current spell.
EDIT: As Colin D mentioned, a rare spell (found at the clock tower during the witching hour, dropped from the skeleton king if killed, and found at certain locations in hell) will replace non-rare spells. Also, when the match advances to the hell portion of the battle you will gain a random non-rare spell (this will always replace your current spell).