That seems to be a "feature" of Firefox web browser, not really a feature of CKeditor. There is a 3rd part plugin that can make that happen in other browsers but we don't have that plugin included in mojoPortal, so it seems an issue limited to Firefox.
From googling about it I found a plugin that can block it, you would have to add the plugin below the plugins folder down under /CleintScript/ckeditorxxx/plugins
then you would have to add the plugin name in the config.extraPlugins
or point to a custom file if you are worried about losing that change on upgrade.
I would think the easiest way to handle the problem with trusted users is tell them don't paste like that. Using drag and drop in mojoportal to add images to the editor should upload the file (unless you are using a very old version), only paste results in base64 and only in Firefox I think.
Hope that helps,