From what I understand its actually a computer programme that does it. Porn sites buy web addresses of forums and message boards and chat rooms and that gets fed into the computer which then goes thru the reg process on its own for all the sites and posts a preprogramed message etc.
The only way round it really you may be aware of. When you sign up for some forums or secure banking etc you have to copy the text from a box at the bottom of a page, a unique and random set of numbers and letters which are different everytime, written in a weird script. That way the computer cant register as it cant identify the letters and cant get access, but people with eyes can.
Although, if it doesnt happen often its not really worth it, and you've got loads of mods