Do mean scuttling your own structures, or de-constructing enemy structure in the gravity well of a planet you've just captured?
The <<<>>> button in the middle of the lower edge of the screen (or the appropriate key combo) will scuttle your own structures.
Deconstructing enemy structures was, IIRC, introduced with v1.32 of Diplomacy.
It is an auto-cast ability of construction frigates/drones.
If a drone has not been given an order it will deconstruct any enemy structure in the well, although you can order the drone to deconstruct a specific structure.
