Put the following code in your config.js file, located in ckeditor root:
config.forcePasteAsPlainText = true;
There's a bug with FireFox which still lets the formatting go through, but there's a patch for it. More on that here: https://dev.ckeditor.com/ticket/8968
There's a bug with FireFox which still lets the formatting go through, but there's a patch for it. More on that here: https://dev.ckeditor.com/ticket/8968