You can use the
bgmp_list-marker-output filter to make the output whatever you want.
function customBGMPListOutput( $markerHTML, $markerID )
return 'your new output here';
add_filter( 'bgmp_list-marker-output', 'customBGMPListOutput', 10, 2 );
$markerHTML is the current markup, and you can use $markerID to lookup info on the individual placemark (to get the title, description, etc).
What's the URL to the map your site? Depending on how your theme is setup, you may need to use a !important declaration in the CSS rule.
text-transform: none !important;
Also, make sure the raw text in the source code isn't capitalized. If it is, 'none' will just let it remain capitalied. You can use 'lowercase' or 'capitalize' instead.
SP reference for text-transform.