Hi,

For now, Soledad theme doesn't support this feature from Tag Cloud widget itself.

You can do that by add more this code at the end of functions.php file - like on this image

\
add_filter( 'widget_tag_cloud_args', 'penci_exclude_tag_from_tag_cloud');
function penci_exclude_tag_from_tag_cloud( $args ) {
  $args[ 'exclude' ] = '21'; // ID of the tag. If multiple tags use comma delimited sting '2,5,36'
  return $args;
}

With '21' is your tag ID - If you want to exclude multiple tags, use comma delimited string. Example: '2,5,36'

Best Regards,

PenciDesign