As stated in instruction manual

This is still the case in 1.12. Redstone will only stop NPCs working the sites (or really providing power to the worksites), but doesn't stop worksite from doing the work if it has power. going to fix it in 1.12.

Which worksite ?
What redstone setup did you try ?
I tested that "shift-placing" on top a lever would apply correct signal.

All farms...did not try mine. Hmmmm...didn't try that. Tried all sides + adjacent blocks but not the top...I had the shafts going into the top of the blocks.