If I understand you correctly, you asked a question & instead of genuine answer, some bozo responded with links which were basically either advertising something or some other unrelated subject?
I am sorry if I sound like I am answering your question with a question, but I wanted to see if the experience which prompted your question was similar to an experience I had. I asked the question: What causes varicose veins? The first person who answered had a different agenda. They provided me with a link which sold alternative medicine, herbs, supplements, etc. I took the chance and checked out the site and there was nothing related to 'varicose veins'. I asked what the 'Cause' was, not how to heal them. The site didn't even address healing them. I reported the answer and unfortunately Yahoo removed my question, so no well-intended soul could give me a honest answer. I consisder myself fortunate because the link could have taken me to some 'porn site' or something equally disturbing.
So, my answer to your question "isn't there a way to stop this?" Probably not. It is basically open to the public and there are those who will try to take advantage of what could be or is intended to be a good thing. Yahoo's TOS says this type of stuff isn't allowed, but again there will be people who will ignore the rules. This is the very reason I don't partake in Yahoo's Groups. If you want spam that is the best way to become a spam harvesters dream.