Seems like some spambots are still registering. I don't know if it's a human solver, if spambots got smarter, or what. I'll try changing "backwards" to "in reverse" when I get the time and see what happens.
Certain bots can easily read written words, even in a picture. Make a hand/paint.net drawing of something and do not use the right colors (ie. a pink dog) and ask people to write down what your drawing is. Bots can hardly recognize a hand drew picture, especially with inacurate colors. We're talking about forum spam bots here, not a CIA secured website, so this simple thing should be enough to keep bots at bay + you get to draw something silly.