Unless your bandwidth bill is tiny, they shouldn't be causing too much trouble. That being said, if you need to stop them before they hit your server (and use your bandwidth), I'd recommend something like
Cloudflare – you can use their firewall to blanket-ban countries or IP ranges, and they'll never get to your origin server at all. Their CDN capability can also help reduce the bandwidth your site uses by caching resources such as images. It takes some custom configuration to make it work effectively (particularly in terms of passing through visitor IPs correctly), but the resources to show you what to do are readily available on this site.