- you can ping client(not the best method)
- you can use application.onDisconnect(objClient) method in your main.asc
- if you have multiple server-side class(other .asc) you can attach an
onDisconnect method to this class

these solutions do not require a lot of code and I think they are the best in
your case(from what I read in your small post)

ex:
myClass.prototype.onDisconnect = function(objClient){
// your code ....
}