Currently there is no API to blacklist or disable a specific Webkit plugin.
Moreover setting the global attribute QWebSettings::PluginsEnabled to false do not prevent the loading of the library / dll.

If the plugin is not compatible with Qt5, for example the plugin libokularplugin.so is linked against Qt4, the application segfault...

An extract of the stack (the full stack is attached):

WebCore::PluginPackage::fetchInfo
WebCore::PluginPackage::createPackage
WebCore::PluginDatabase::refresh
WebCore::PluginDatabase::installedPlugins
PlatformStrategiesQt::getPluginInfo
WebCore::PluginData::initPlugins
WebCore::PluginData::PluginData
create	PluginData.h
WebCore::Page::pluginData
WebCore::DOMPluginArray::pluginData
WebCore::DOMPluginArray::canGetItemsForName

There is already a function PluginPackage::isPluginBlacklisted() to do that, but everything is hard coded...
There is no public API, and the private API (QWebPluginDatabase) does not provide a way to prevent the loading of the plugin library