In the proversion in Admin go to Setup>Configuration>Plugins>payment and disable the 'free' plugin. I don't know about the free version.
I have been trying to disable the Free option, but it always turns itself back on. I wanted to try out the CCdemo, but Free always comes back along with it. CCDemo does get enabled, but I am never asked for any CC information during signup, as if the whole thing is still Free.
1. This is impossible to disable free plugin in latest versions. 2. You get this situation because your product price set to zero. Go to aMember CP -> Manage Products -> Edit product and set any price for your product.