Add blacklist

[ohrensessel]

It would be helpful to add a blacklist (MAC address based) to the form, maintainable via the admin panel. When deleting a node via the admin panel it would be nice to have the choice to add this node to the blacklist.

We often remove nodes from our network that are not in Hamburg (or not even near). To prevent such nodes from signing up again without geo coordinates, a blacklist would be helpful.