Yes i highly recommend this plugin http://www.oxwall.org/store/item/583 if done correctly you will nip that problem in the bud.
To do this correct dont just take the default questions and answers, redo them.
1. First remember that if you are going to do other languages then have the answer for those languages as well. If not and you just plan to use english, then use only english answers. Remember to do both text and numeric answers for questions that applies to, example 12 and twelve and Twelve (in case they do the uppercase T).
2. Do questions that they have to be able to use logic, reason, or intellegence to answer. Math answers can be used but stay away from things such as 1+1 = the bots have become alot smarter and many of them can answer simple math questions.
A good question would be something like what is the color of the background of this site, or what is the color of our logo, or what is the 4th item on our menu.
You can even add a special image to the front page of your site lets say a small circle or square someplace that is unobtrusive but they can see easily. And ask what color is this circle image. Then change the image once a month or so and correct your answer accordingly.
The point is to ask questions that a human would have to be sitting right there looking at your page to answer. I have even gone as far as giving them another url of another site i have or another site entirely or even another page on the same site and asking them what color is the image on that page. That way they have to open a browser, go to the url to see the image and then come back and answer.
Another type of question is a missing letters question. Such as, plural for goose is g _ _ se.
Or one step further have them enter just the missing letters only. Such as, (please enter just the missing letters) A boat is to w_ _ _ r as a airplane is to the air.
They say that bots are becomming able to reason and even some of them able to refer to google on their own for the answers or atleast keywords to try, i dont know if i believe that but i will say that those idiots that build these bot scripts have nothing better to do and its a huge business financially, so where there is a will there is a way i suppose.
Also rememer that once any bot gets access to a site, they pass the word to other bot scrips like cock roaches and that is probably why you have so many, all the bots got the access secret and hit you like crazy.
Protecting against spam is actually working its way up to congress as it costs companies tons of time, manpower, and the big thing money and is counter productive. Goverments are beginning to really take this as a serious risk now to the economy and it is slowly getting the recognition it deserves. And rightly so!
I hope this helps you...