i'm sure there is a way to do this. Try to work with touchID's every touch has his own ID. for a quick and dirty solution i would try to do something like this:
global variable "singleTouchID"
on touched "block" -> "singleTouchID" = touch.ID
if touch.ID = singleTouchID -> destroy block
now i dont know how fast construct reacts, but if your lucky the player will only destroy one of the blocks if he touches multiple ones the same time.
i think there is something like touch.count too .. if i remember correctly .. then u could do stuff like:
on any block touched -> if touch.count = 1 -> block - destroy
just have a look at the touch expressions and properties ..
feyninja was faster .. i'd go with his solution.