trigger: ability cooldown

For "Unit - Reset ability cooldowns for (unit)" is it possible to reset the cooldown for only a certain ability?

Short answer, no. If it is a unit ability you want reset, it should be simple enough to remove the ability and add it again.

removing and adding the ability doesnt work. It just permanently removes the ability.
Im using this as a workaround for an ability that i want to only be able to cast on certain unit types. If the unit type of casting type not equal so and so i display a message and i want to reset the cooldown. The thing is the caster has other abilites that i dont want to be reset simultaneously.

Change allowed targets of spell...

You can try to have a trigger that runs whenever a spell begins casting and if its equal to spell x and target y then order the unit to stop... but I don't know if that would stop them.

I believe it would: the "unit begins casting" event can be interrupted (and in that case, no mana is spent and no cooldown time started); the "units starts the effect of an ability" event, on the other hand, cannot be interrupted anymore as the spell is already cast at this point, so this is normally the most usefull event when it comes to spell triggering (unless, like in this case, you want to interrupt the spell before it's cast.)