How do I make it so that when I go offline on aim, it won't send my IMs to my phone?

I registered my cell phone on aim a few months ago. Then I liked to have my IMs forwarded to my phone. But now I think it costs extra. So I tried to go to my settings on my computer on aim and delete my mobile device, but whenever I go offline it still forwards my IMs on my phone. How can I make it so it doesn't forward them to my cell phone?