Guys, deleting the css file in an epub is complete overkill if you just want to remove the font.
What you should do is explode the epub using calibre. edit
the css file using your favorite text editor (yes, notepad will work, although I wouldn't recommend it) and search for and delete the following...
Any instances of
some font related styles
The @font-face rules will generally be grouped together in the CSS, and you can do a simple find/replace using wildcards or regular expressions for the font-family rules.
Then rebuild the epub using calibre and you're done.