[Nitram Draw]

I would propose a different type of fix. Instead of preventing the opening of a worm hole why not remove the ability to close worm holes, once they are built the stay. This way you could use worm hole offensivly but not defensively. You could secure your system if you wanted but couldn't change your mind later.
It is usually easier to get the AI to act offensively than defensively.
Another alternative would be to make opened worm hole temporary, the close a turn or two after opening.
The whole idea of securing a system by opening a hardcoded limit of worm holes seems very gamey and should be avoided IMO. Just my 2 cents.