Scrap the bloody facility if you *must* take down that fleet, and it is in one of your systems.
If you really want to live in peace, you could only leave one wormhole leading to other systems, and build a *nasty* fleet right on that wormhole (everything you can think of, and then some more). No more wormholes can be created once there are 10 in a system, and that facility also prevents wormhole opening.