I've had luck with a putty knife and propane torch. But expect wear and rear to any paint underneath.

THIS!

I have done it this way with two vehicles. The quickest and cleanest way to do it. The stuff comes off in large chunks.

Ziebart is different than undercoating in that it is more like a heavy film coating. Sometimes it can remain gummy for decades. When I found it on my car I was happy to leave it in place. I would agree with those using gasoline or something like it to soften it. I have never seen it thick enough or dry enough to scrape or chisel it off.