The solution is to paste plain text, then format. Sometimes it is beneficial to copy then paste to a plain text editor, then you can copy and paste to the RTF. The forum is chock full with similar posts. You can also look at using a modifier to paste.
To ensure that no unwanted formatting is pasted, use the keyboard shortcut to paste unformatted text (CMS+SHIFT+V on mac — CTRL+SHIFT+V on windows). This will strip the text of all formatting including bold and italics. Be careful though, it also removes links.
More info about working with the Rich Text widget is available in the university.