Put this file in the modules folder where you installed With Fire and Sword. Open it, select Ogniem i Mieczem (with fire and sword in Polish, as the game is named after a Polish book). Import. Click on items, then editor. In column AT (remove item on use), delete the variable in the column. To find grenades in the file, use ctrl + f.
It can be found here:
http://forums.taleworlds.com/index.php?topic=117997.0