This was a bit harder than I originally thought. Follow these steps:
1. Locate qa-wysiwyg-edidtor.php file
2. After this line:
" language: " . qa_js(qa_opt('site_language')) . ",",
Add this line:
" forcePasteAsPlainText: true,",
All you need to do is to replace the folder qa-plugin/wysiwyg-editor/ckeditor with the folder ckeditor in the downloaded link. In case you've made other customizations to your CKEditor, of course, you will have to apply them too.