See this video for better explanation: http://screencast.com/t/vKhnodrD When you go to a serial that you have entered a subscription end date and delete an issue it generates the next issue. If an end date is entered it should not generate anything.
I think there is no way to prevent the generation of a next issue, but if you end the subscription for real, you can close it. This will hide the last 'expected' issues by setting them to 'stopped' and you can resume the subscription at a later point in time.
I believe the current behaviour is correct. Please reopen with more information on the use case if necessary.