Sorry for my delay!
Unfortunately it's a bit hard to stop spam completely, there are many talented programmers that write spam bots to automatically spam the contact (or any kind of) forms on the web.
Some are very sophisticated and they are able to read the captcha images.
The first thing you can do is to use an unconventional captcha image like this one:
http://www.hamidof.com/Downloads/PHP...aptcha_Script/
Which is freely available for download.
There are also other ways to deal with spam; some of those spam bots, spam your form with a message full of links.
The links are not in HTML but BB code like:
Title
If you know PHP, you can write a piece of code to detect and reject these kinds of spam, you can read a discussion about this technique here:
http://talk.code-head.com/showthread.php?t=596
I hope this helps.