I'm afraid this isn't possible from the web UI as of 10.2.2. The best way to unmanage more than one node at a time on demand is going to be: Settings > Manage Nodes, then check the nodes to be unmanaged, and click "Unmanage" (I realize you probably know this already).
For scheduled mass-unmanaging, SolarWinds' answer is the Unmanage Utility (http://thwack.solarwinds.com/community/solarwinds-community/product-blog/blog/2010/04/13/scheduling-unmanage-task-for-nodes-interfaces-and-applications). It leaves a lot to be desired and is manual in and of itself (if you add a node to Orion on the web, you'll need to go add it to your job(s) in the utility).
I'd love to see more development on the Maintenance Mode / "Unmanage" front within Orion. 'Been asking for it for 2+ years.
I'm aware of the way to unmanage nodes but thanks for the reply. It would be very nice to unmanage based on the groups we create to speed up the time to unmanage devices during a maintenance window. I would like to see this featuer in a upcomming releast of NPM. An even better option would to be able to unmanage nodes using powershell.