So you can set the devices as unmanaged, but they will still show in the website, just as unmanaged instead of down/up. There is no way to "hide" them till they come back online unless you want to remove them from a users view limitation.
although you cannot remove them from maps, but you can remove them from resources on different pages such as a Problems Overview page. Under the resource select Edit and under the Filter Nodes field put in status != 9
9 is the number that represents the status 'unmanaged'.