One way to stop the Arc application server (i.e., app.arc or asv) is to run the server in a separate thread and kill the thread when you want to stop the server, like so:
(= app (thread (asv)))
and, to stop it:
(break-thread app)
This will allow you to get back to the Arc prompt without pressing stop in DrScheme.
As for your second problem, the reason you are having trouble is that you are typing in the URL for the hello operation. You need to use http://localhost:8080/hello2 instead where the portion after the / matches the name of your defop.