Find people recently added to Google+ circles

This is not currently possible. I have browsed the list of Chrome extensions and userscripts and there isn't anything that accomplishes your task. Best to wait for the API to reach read state.

Also popular social networks do not handle their import this way. Mass subscription sounds like something Google would try to prevent in the same way Twitter prevents that service.

It is proper to do nothing and let them circle you. Think of it like Twitter when a brand follows you; it's not a problem. I would not block them as standard practice (only block and/or report when appropriate).