/System/Library/CoreServices/Finder.appBe sure to choose the correct Finder (the one that's over 4mb and version 10.1.2, not the 4K dummy Finder). Now do a "Show Package Contents" (control-click) on it. Navigate to
Contents/MacOS/FinderUsing XRay, get info on the file Finder. Now simply turn off the execute permissions for the owner, group and world. That's it.
Whenever you download a file from OmniWeb, the Finder will try and launch, but it won't succeed (I have the launch animation disabled, but the little arrow comes up once and then goes away). Now, your Finder will only launch when you want it to. To get it to launch, just give the permissions back.
[Editor's note: I haven't tried this myself, and if you don't have XRay, you'll have to use the Terminal to access these settings.]

