If you are pasting in content from Microsoft Word documents, use the paste from word feature.
To use this, click the word clipboard (2) icon and paste the content into the prompt. This will safely paste the content into the page and remove code that might get copied over from word.
If you are pasting content from other websites or data sources, use the paste as plain text (1) feature. This will safely paste the content into the page and remove any code copied from the source. One drawback to using this is that it will strip all html formatting. After pasting the content you will need to reformat the content.