WoW:API SpellStopCasting

Stops the current spellcasting. Doesn't work for channeled spells.
Returns nil if it didn't do anything (no spell was being cast at the moment), 1 if it did stop a casting.

Can disable autorepeated "spells", like hunter's Auto shot.

A recent post from the WoW main boards suggests that you can link two abilities to fire from the same macro press (and that this move is legal) as long as one of the abilities does not trigger the global cooldown.

This is done with the following simple setup.

/cast Spell Without Global Cooldown
/script SpellStopCasting()
/cast Spell With Global Cooldown

It's been tested to work with Nature's Swiftness (Shaman/Druid) and also Divine Favor (Paladin), but should work for any ability that does not have a global cooldown attached.