how to clear the margins of the corners of a checkbox in a listbox

Add this code to your ListBox:

<ListBox.ItemContainerStyle>
    <Style TargetType="ListBoxItem">
        <Setter Property="Padding" Value="0" />
    </Style>
</ListBox.ItemContainerStyle>

This will set the Padding of the ListBoxItem container to 0. The result looks like this: