i set category not to show pagination in some categories in site. that cause to display the top and bottom line with out the page numbers. how can i remove lines from categories that i dont want all all pagination?

i would use firebug to find and hide the offending elements.
next, if you change k2.css it will most probably be site wide
I would create an override and use css4k2 to create a style just for that category with the necessary changes to YourOverrideName_style.css