With the Android Market you are given a grace period of 15 minutes. If you realise that you do not want the app (or installed the wrong one or whatever) and uninstall it within 15 minutes you get an automatic refund. If you do not do that within 15 minutes then you are deemed to have chosen to keep the app and there is no refund.
If you are referring to Norton Mobile Security you initially download the Lite version to try, and ensure that you know what you are getting. Then if you choose to upgrade and pay for the extra features I seem to remember that there were several reminders that there are no refunds.
If Lite was downloaded then Mike is right that there would be nothing to be refunded. If you did the puchase and would like a refund then you can contact support:
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