One of our sites is getting spammed about every 25 minutes with about 10 spam comments. We have ReCaptcha enabled and I have tested it to find it working correctly. How can we still be getting spammed?

One thing I noticed is that we have one real comment from user on one of our K2 items. All of the spam comments are made on that same K2 item. Is there some kind of exploit where bots can use existing comments to throw in their own? I just unpublished that comment for now to see if that helps, but any assistance you can offer would be greatly appreciated.

Also, we're running Joomla 1.5.25 and K2 1.5.4

They keep getting bombed with spam on that one item. I have unpublished it so I'll see if that helps. What would be even better is if whatever is being exploited could be brought to light and fixed. I can't submit a comment without passing ReCaptcha so is ReCaptcha broken or K2?