In GL forums, only one google ad is shown to non-members above the latest plug-ins/scripts downloads that I created/edited.
In Lottery forums, ads are only shown on custom lottery tools pages that I created to everyone.
So during regular forum conversation there are no ads shown at all (to anyone).
Plug-ins/Scripts written by other authors default to having no ads (at all). If however, they wish to show an ad (to help GL earn ads money) to non-members they can add only once per page the tags [ads][/ads] (above the latest script/plug-in version download [attached file]). But it is completely up to the plug-ins/scripts's author's discretion.