I want to delete contact from code side. And I have only this URL. mysite.com/Newsletter/unsubscribe/default.aspx?p=$WLopzs9vAxwtOxCi8TqSbWytIULA-K0D By p querystring I can get contact information but didn't understand how to delete this contact. Please help me.
It's not possible to delete a Contact on the Content Delivery side. It must be deleted on the Content Manager side.
You can filter for Contacts that have unsubscribed and then delete them, perhaps in a service if you need it at regular intervals.
Be careful, though. Deleting a Contact removes all of the information forever; there is no way to restore it again. That might be what you want (for legal reasons, for instance) but I just wanted to make that very clear.