If you put the balloon and pinned sprite in a 'container', then when you use destroy balloon it would destroy the related pinned sprite that was created with it.
THANK YOU, 
lionz! This worked perfectly.
> You could also use something like on balloon destroyed, pick pinned sprite overlapping or nearest to the balloon and destroy it.
I don't think I'll need this for this project, but I'm curious, how can I do that?