Is it possible to set up a whole entire fortrees of ships with gravity wells and place them all with people(I read the rules about them) assuming this is before combat (in which case they would just destroy the ships) but could I trap ships in this one area of space and prevent them for ever leaving unless I choose to?
And I am well aware of the time effort and money this would cost me.
They can fly past the ships with sublight or even fight their way out and are you really willing to waste a really HUGE ammount for building a HUGE number of such ships, while you could just disable the fleeing ship's engines?
There is no point. Just wait for combat and ion the damn ship.
Though, closing a system with gravity wells is quite a retarded idea. Because: nobody can get out. NOBODY. Friend, foe, neutrals, they're all stuck. People who come into the system to conduct business can't leave. If you want to piss off everyone, then it's a good idea to do it.
I am not familiar with the operating of Interdictors, but I assume it is possible to switch their gravity field on and off. If so, you switch off if someone leaves, and switch on a second after that. Just a matter of coordination.