You spawn a new blue circle and you want the previous blue circle to be deleted?
If you only have 1 circle at any time on the screen, then simply destroy it first and then spawn a new one.
If you have many circles and need to delete only the last created instance, you can use a variable to store its UID.
When you need to delete last instance, pick it by UID and destroy.
When you spawn a new one, save its UID to that variable.