To control bandwidth, we use a bandwidth throttling system. It continuously monitors your usage as it serves your pages. If the trend is that you will exceed your quota, the throttle will start to slow down when serving your pages in an effort to keep you within your site's quota. If this slow down becomes too large, then the server will refuse connections and display our 408 error page until your site's usage trend falls back within your quota. It will then start serving your pages again.
Truth be told, there really is no such thing as unlimited bandwidth.
If any do claim to offer completely unlimited bandwidth, they are either lying, grossly inexperienced, or both. What reputable hosting companies mean by unlimited bandwidth is that they will make available to you however much bandwidth that you require. What they usually don't tell you is that there is an extra cost for this.