The contact form on my website receives some spam occasionally.<p>It's always a meaningless jumble of urls.
It is always of this type (I've replaced the meaningless jumble with some of my own in case the urls are valid):<p>ty8769 <a href="http://iduqfhuerhferif.com/">iduqfhuerhferif</a>, [url=http://powjrjie.com/]powjrjie[/url], [link=http://oproprktt.com/]oproprktt[/link], http://pesdukiqwro.com/
What the other posters said. I would add that if this is disruptive you could try using a visually hidden field with a special value. If the field gets tampered with, you can flag it as likely spam. This works because the bot just traverses the dom and fills in fields that look like comment boxes (textareas) with spam.
They're list building by submitting to random forms . Then they go back and try to see if the comment when through the form and shows up as a comment on the website. If it does, then they know they can spam that website in the future.