I'm not sure it's a good idea to block it, since you give them a piece of data (this user blocks canvas fingerprinting). There's one called CanvasDefender, but from what I read it's noisy with popups.. I would like something like could periodically change the fingerprint, though I'm not sure if it's really feasible. (As far as I know, the website can also change the "canvas challenge", so it's not like you can just send a particular fingerprint per website for a while. For example, they could send 10 different canvas detectors, and if you send the same response to every one, aha.)