Use the "is pinned" condition from the pinned object.
Then on your ontouch event add the condition bullet is pinned (and invert it so it basically says bullet isn't pinned)
Now its possible I miss understood you when you said it continues to rotate a little bit once it hits.
If by that you mean you had your bullet rotate a set amount of degrees per tick towards touch.x and touch.y
then you'll have to add an event on collision that instantly finishes the rotation or simply stops the rotation.
Depends on what you're using to rotate the object (the sprite angle, the bullet behavior angle, ect.).