The best protection against spam, imo, is a challange-response question.
I have an image based one here -
Blaa.ie Forum - Registration Image Verification - Obviously that one is slightly more complex than required for yourself but for you to implement something similar, a text based one for example which asks 'what's the capital of Ireland?', shouldn't be too hard.
If you install a popular open source capatcha type script the chances are that spammers will target it. And of course bots can read capatcha images but they don't know what the captial of Ireland is.