We send updates. If a server on the list isn't updated after waiting for 60 seconds, then that means the server is not running any more.

Then (if I remember correctly) every time someone attempts to download the list of servers, we run a code to check which server entries in the list have not been updated during the past 60 seconds, and those are deleted before returning the server list to the one who requested it.

May not be the best method, but it works great for us.