Currently there is no good method in SharePoint for deleting all items from list. But there are solutions that need this kind of functionality. During one of my projects I wrote extension method for SPList that removes all items from it.
public static void Clear(this SPList list)
var count = list.ItemCount;
for (var i = count - 1; i >= 0; i--)
var item = list.GetItemById(i);
Feel free to use this code. Also let me know if I can make this code better somehow.