A well configured space will allow the electrics that have units serving as Bax, to be masked from audience view with a border/teaser. If done correctly, and if the theater sightlines cooperate, your back light does it's job of lighting what you want lit while not being seen. This is a factor of front row seating distance to the performance area, enough pipes to hang a border in the right place as well as being flown to the right height, and electrics well placed as well as flyable.
It's sometimes a compromise, and in some spaces, impossible to correct and in some instance, desirable to see the fixtures.
I have this problem in my theatre. We have no fly system, and our lighting grid is only 16' above the stage. Any back lighting positions focused on the prescinium of the stage leak into the first couple rows. The rest of the auditorium can see the source of the light, so I always use barn doors, and usually extend the barndoors using black foil.
In your situation, you might want to try using a S4 insted of a fresnel. Then you can just shutter cut off the edge of the stage. If your worried about the amount of area that you light with a S4 you could use a 50 degree.
As so many people have said, a lot of it is about theater design. If you don't have good design, then moving your curtains, well placed ellipsoidals with shutter cuts, top hats, and tricks with barn doors and black foil are all good options. One point I want to toss in is that you can have a fairly high angle and still get an ok back light look. If nothing else works, try moving the instrument position closer to down light. It's not perfect but it might help.
The S4 would cause the same problem. It's not an issue of "I have light bleeding where I don't want it", but more of an issue of our stagethrust goes out INTO the audience, so to make sure I have adaquet hair light when the person is standing on the thrust, the 20 seats around and infront of the thrust get lighted. It's a theatre/seating design problem. My have very large barndoors on my fresnel so it's not that.
Yeah, top light would probably work for the people in the room, but it doesn't look great on video. The camera sees the bleed of the top light onto the forehead, chest, legs, hands, et - and video quality is a priority.