Few days before I was having client requirement to unassigned around 50K + items from specific sites. There were several help available on web one of is from MVP Mohood (http://mahmoudsaadi.blogspot.com/2013/11/delete-item-site-assignment.html) . I followed his sql query to delete all those items from sites.
But recently I found new feature in Dynamics GP 2013. From Navigation-->Click on Inventory --> Click Items. You have option to add filters base on your requirements. Check mark items you want to unassigned from sites. Click on Unassigned Site icon. Select Site and click Unassigned.
You have option to to add site with same screen.