Go to bottom menu, last tab - disable publisher styles, to test this.
This breaks many other styles as well. If you need them back, enable again, then create a Style Tweak to override the margins. For example, with:
p {
margin: 0 !important;
}
Or any other margins (read CSS documentation).
Styles Tweaks are in the manual:
http://koreader.rocks/user_guide/