And of course you can throw black holes into black holes as well, and extract even more energy. The end game is when you have just one big black hole, and nothing left to throw in it. At that point you then have to change strategy and wait for the black hole to give off Hawking radiation until it completely evaporates.

But all these things can happen later - there's no reason for not going through a paperclip maximization step first, if you're that way inclined...