Is it possible to generate video on the fly containing random characters and numbers and use it instead of the still images? or may be generate a .gif animations (with characters dancing up and down in a colorful way :))?
I assume that can slow ocr (forcing it to convert the video to still images first then do the ocr) but i don't know about video, may be it is possible to make the video gives garbage when converted to still images.. if you google video character recognition you get a lot of hits but i don't think spammers capable of developing vcr for their needs ;)..