As far as I know bots, even advanced ones, have an extremely hard time with dotted fonts. Just change the Captchas to use some really dotted or unique fonts where the letters are made up of smaller elements that don't connect. You can extend the life of Captchas for a few more years.

http://fontspace.com/malwin-b%C3%A9la-h%C3%BCrkey/merkur

http://fontspace.com/honey-and-death/dotline/8617.charmap

http://fontspace.com/bythebutterfly/bubble-bath

http://fontspace.com/jecko-development/jd-lcd-rounded

Eventually, the spammers will make a bot to analyze the distance between dots, group them into letters, and the race will be on to use other methods. I see this as a never ending virus/immunity battle. We're pretty much at the end of Captchas. Other methods like mouse movement, surfing time, scrolling, etc... can all be mimiced as well. Computers can or will be able to simulate humans very well, even our imperfections.