You’re still not quite catching on. If a forcefield is placed on a wormhole, enemies will be pushed OUTSIDE the forcefield, not be stuck inside it. Also if they try to travel through a forcefield, they will be denied entry and thus have to travel around it.
So any enemy entering the system "inside" a forcefield, will immediately be bumped outside the forcefield, away from the wormhole. They do not get "stuck" inside the forcefield unable to get out.
The idea that was used back in the day (that I think was nerfed) was to place four forcefields around a wormhole, like so:
FF FF
WH
FF FF
To leave only a small area around the wormhole. The FFs would overlap and create a barrier that ships couldn't pass.