I recall there being a thread devoted to a rather simple opt-out option for pop unders. It's something that I ran on a previous computer, but that computer is now spare parts, and I'd love to run it on this one.
However, I can't seem to find said thread. I've done a Google search on The-W. I've perused the Internet & Computer forum here. I've even dug through CRZ's World, because I think he mentioned it after it had been around awhile.
My girlfriend's parents gave me their old Mac Mini (with a dead hard drive) after declaring it surplus and deciding it would make for a nice plaything. It's one of the earlier models (A1103, if that helps), back when they were using PowerPC processors.