Blah! I never use cooking spray for that very reason! Cooking spray has water in it, in addition to the oil, and there's something about the water not heating as fast as the oil, and turning gooey -- I don't quite get the details, but I won't use it on cookie sheets.
Anyway - sounds funny, but I've had success soaking things in water with a fabric softener sheet. It loosens up some of the crud. If all else fails, maybe you could try something on the less-abrasive side, like Bon Ami?
"In all our woods there is not a tree so hard to kill as the buckeye. The deepest girdling does not deaden it, and even after it is cut down and worked up into the side of a cabin it will send out young branches, denoting to all the world that Buckeyes are not easily conquered, and could with difficulty be destroyed." - Daniel Drake, 1833