Given the sheer size of constructed worlds they ought to require something as huge to destroy them as to create them. Maybe not even a single component but a collection of bases just like construction. In other words, if you really want to destroy one you have to capture it the hard way first.
Being able to sneak a planet-killer in with a cloaked ship would be just too cheap.
[This message has been edited by Baron Munchausen (edited 07 March 2001).]