Why doesn't the "Invisible to Everyone" status prevent you from getting spam instant messages?

I recently started back using Yahoo IM. I have alot of contacts in my address book, so I almost always have my status set to "Invisible to Everyone" so i don't get constant IMs from my friends and family. The problem is that I get spam IMs from random people. If I were to leave my pc running Yahoo IM, I would have 25+ spam IMs from people I don't know. Is there anything that I can do?