Ahh, right, I see.
Well, I'm sure there's a better way of doing it, but here's the first thing that springs to mind.
I'd check for collision, and if "active" on the picked instance is 0, set it to 2, then use for-each-object to loop through all the instances, changing every 1 to 0, and every 2 to 1.
By the end of the loop, you'll have all zeros except for one instance with 1, which is the one you collided with.
Sounds like it should work.