Those are not unwanted characters. Those are actually the characters you do want. They are displaying incorrectly because of the wrong encoding being used.
In the header right after the title (if needed), I put the following...
Code:
<title>Cat & Mouse</title>
<meta content="http://www.w3.org/1999/xhtml; charset=utf-8" http-equiv="Content-Type"/>
I cannot speak for the current Sigil, but in a pre-0.7.x version, when it got the code page wrong, it could convert things like your quotes to those sorts of characters and if you saved, your screwed.