For this first disable relaying and also for some time stop the default virtual smtp server. Also right click on Current Sessions and say Terminate All. Also try and delete all the queue by doing right click and then say delete.
Hope that this works for you as it worked for me as I had also faced the same kind of problem and i was able to do it.
If you want to open relay then open it for a specific IP only.
>From: "Kalai Selvan"