Hi,
This setting is the DEFAULT setting for new menu items and modules.
For existing menu items and modules, you need to edit the menu item and module via the Joomla menu / module manager. And under the "Products options" tab ("HikaShop options" tab), you'll find an add to cart button display setting which you can turn off.
Regarding the CSS, that's because you need to specify a parent element class.
Instead of just .hikacart{ display: none; } you need to do something like that: .hikashop_products_listing .hikacart{ display:none; }