You can't entirely remove a spell, as far as I know, but you can modify it to require impossible path combinations, such as both holy and unholy.
Try the following:
code:
#selectspell "Breath of Winter"
#path 0 8
#path 1 9
#pathlevel 0 9
#pathlevel 1 9
#end
That'll make the spell require a unit that is simultaneously a level 9 holy and unholy priest. That ain't gonna happen...ever! As a result, this spell is uncastable.