Consider this screenshot: gyazo.com/31fbbaece7189597e100bdf67ba274eb
As you can see, reducing the previous level to 0% will return all the green and red bars to their neutral value (notice the very small lighter shades in the bars at 2% policy strength), which I imagine is what cancelling the tax would do. Only cancelling it costs 30 political capital while lowering it to 0% strength costs 10, and I get ~3 billion income to boot. I believe this game mechanic is broken!