You don't want to use the
Stops casting Event. These Events work in stages:
Stage 1:
A unit Begins casting an ability. The unit has begun playing it's cast animation, mana cost hasn't been spent yet, cooldown hasn't started yet, and none of the effects of the ability have actually executed. The time spent doing this is tied directly to a unit's
Art - Animation - Cast Point field. This can be interrupted with a newly issued "Stop" order or a disable (stun, silence, etc).
Stage 2:
A unit Starts the effect of an ability. The unit has finished it's cast animation, the mana cost has been spent, the cooldown has started, and the
effects of the ability SHOULD take place. However, you can interrupt the
effects at this stage by removing the ability being cast from the unit. Note that doing this will still spend mana even though nothing else happens.
Stage 3:
A unit Finishes casting an ability. This is useful for detecting when Channeling abilities finish. Unfortunately, it loses track of most ability-related Event Responses like (Target unit of ability being cast) and can be completely skipped if you issue an Order after
Stage 2 has taken place (unless the ability is channeled). That's why you shouldn't use this outside of it's intended purposes!
So all
3 Stages can be interrupted in one way or another.
Stage 1 can be interrupted by normal game mechanics like orders and disables,
Stage 2 can be interrupted if you remove the ability, and
Stage 3 can be skipped entirely if used incorrectly.
A unit Stops casting an ability will depend on what happens. If everything goes as planned then this will occur after
Stage 3. Otherwise, it will occur when Stage 1 or 2 are interrupted.
With all of that in mind, you'll want to use this trigger:
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Events
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Unit - A unit Starts the effect of an ability
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Conditions
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(Ability being cast) Equal to Boulder Hurl (Fel Orc)
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Actions
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Wait 0.01 game-time seconds
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Unit - Remove Boulder Hurl (Fel Orc) from (Triggering unit)
This will occur after the effects of the ability have executed and will avoid the aforementioned "remove on cast" interruption thanks to the Wait.