CanvasBlock is a GreaseMonkey script that block HTML5 canvas elements like FlashBlock.
You can install it from http://userscripts.org/scripts/show/74216

Caution: Please do not use it in daily life, it is experimental. Blocking a canvas element may crash your browser. Moreover the HTML5 tags are part of the web and I don't think blocking canvas or any other tags brings any real benefit to you.

It is named CanvasBlock but it does not only block canvas tags, but also video and audio tags.

The blocked tags are replaced by a line of text, "Click to show xxxx". You may click on it to get back the blocked content.

You may test the script at 
http://calinmocanu.webs.com/the_canvas/fish.htm
http://www.mozilla.com/en-US/firefox/video/