I want to add a simple Contact form to my web site so that customers can contact me easily.
You won't need to reduce spam cause the messages are not published on the website. A lot of spam is posted on forums and blogs because this will reach a large audience of viewers and bots.
For a private contact form, spam is ineffective, so you won't have to worry about large amounts. The few spam messages that you will receive can effectively be filtered with a spam filter on your inbox (for instance using gmail or yahoo), especially since the incoming messages are plain text without images.