Yahoo IM - When forwarding IMs to my iPhone, why am I getting spam text messages when my settings block users not on my friends list?

When using Yahoo IM on my laptop, I block users who are not on my friends list due to lots of spam IMs. Works great! THEN, when I go mobile (forwards Yahoo IMs to iPhone as a SMS text message) I have gotten a couple nonsense spam texts from people not on my friends list. They get filtered on my computer no problem, so why are they being sent to my phone?