Tested on OS X - the small attached application should fail to load the plugin for the quick time movie - this is because the Qt libraries are 64 bit and the QT plugin 32 bit, and is not the bug. Not tested on other platforms.
The application also sets QWebSettings::PluginsEnabled to false, so it should not be loading the plugin anyway.
After quick inspection, the WebCore::Settings does get the pluginsEnabled flag set correctly, but it isn't being read when the plugin is being loaded.
Attached patch causes the FrameLoaderQt to check if plugins should be loaded, and returns a no content flag if not. I don't think that this is the right place to check the flag, but am not sure where the check should go - would appreciate advice from WebKit expert.