You will need to update the gallery.js file for the gallery you are working with. For example, if you are using the Ribbon+Fullsize, you would the edit galleries/ribbon_fullsize/js/gallery.js file. On line 59 you would change it to remove the : part so it looks like this:
thumb.store('title', parts.length > 0 ? parts : ' ');
The line number will be a little different for each gallery, but the change should be the same.