I see nothing wrong with installing speed bumps or even a
gate across the road. It would not have to be locked, but she
would have to stop her car to open & enter & stop again to close
the gate back.

I'm sorry Fox-Gal, but you are being way to nice to this person.
She & the boy friend are showing no respect or regard for you
or the animals who live there. This really ticks me off that it's the
poor kitties that have to pay the price for her stupidity.