I've just done some playing around with my epub files and I can confirm that you don't need to delete the style sheet to get your ePUBs working. It is probably the fastest solution but here are my findings.
The problem lies when you have a font-size style applied to a <P> tag. If you take the font-size off the <P> tag and put it on a <SPAN> that lies within the <P> tags in the html it will work. This is not something that you can just press a couple buttons and be done, but I believe this means that you can truly FIX your ePUBs to have them display 100% on your KOBO.
I don't know if Calibre has an option to use <SPAN> instead of <P> but maybe there is something that can be done similar to that.