How to hide the (-) sign from facet filter block?
There are a couple of theme functions you can use to work with the display of these filters.

theme_facetapi_deactivate_widget($variables)

theme_facetapi_link_active($variables)

In your case, you could place the following in your theme's template.php file to edit the display of the deactivation 'button' of your facet:

function THEME_facetapi_deactivate_widget($variables) {
  return '';
}