I saw a video of a modeler who used a ceramic scraper to cleanly remove flash on a part. It was impressive, but the fancy Japanese scraper was expensive (about $35, with just one blade). I have a ceramic strip in my knife sharpening tool. It's bulky, but I used it on some flash and... it worked! So I ordered some ceramic blades from Amazon. One is a set of #11 blades, the other is a blade for the Olfa snappable cutter (both are white ceramic):

The edges are ridiculously sharp and can easily cut/remove flash and cut through plastic with no problem. The blades are fragile, but the results are impressive.