Once a black hole is formed, is there anything other than Hawking radiation which shortens its life? Hawking radiation is supposed to very slowly evaporate a black hole (terms and conditions apply :] ). 
Apart from Hawking radiation, is there any mechanism or effect that can make a black hole cease to exist? Or once they are formed are they expected to exist in this form "forever"?
 A: To find a process that can cause a black hole to be destroyed much faster than the time-scale for complete evaporation via the Hawking process, we can consider the Big Rip model. According to this model the universe could end quite soon due to ever faster expansion of the universe. This means that not even black holes could survive this process. 
Then, even if the Big Rip isn't actually going to happen, we can study how during a hypothetical Big Rip a black hole would lose mass and then see if this can also happen under different conditions. It has been shown that in a Big Rip scenario, the mass of a black hole will reach zero due to accretion of phantom energy, see here and a more rigorous argument is given here where it is shown that the mass will actually reach zero before the Big Rip is reached.
A: Bear with me here.  Suppose I have a rotating black hole X. Then the region of space just outside its event horizon is rotating.  Not just orbiting objects, but space itself is "frame dragged" by X.  So tidal forces mean that if I orbit X just outside this radius, I'll experience a force trying to spin me.  I mean, presumably the frame dragging has a derivative one way or the other, which will try to spin me, as well as the more obvious trying to stretch me.
Okay, so now let's move a small black hole Y into this region, give the tidal forces time to spin it up (it can't stretch), and then move Y away again (at this point, we're firmly in the realm of science fiction, but there's no physics being violated, just engineering).
Once we've spun Y up, it now evaporates faster due to the Penrose process.  Rinse and repeat.  Or leave Y there to evaporate.  Whichever you prefer.
Let me be the first to point out that "in practice" (whatever that means in this realm) the increased radiation is vastly dwarfed by the energy that we put in which gets sucked into Y and makes it heavier.  And my guess - but it's only a guess - is that you can't actually win here, even in theory.  Some higher principle probably says that you have to make Y heavier by more than you gain from the spinning.  But I've never seen such a principle.  Basically, the aim is to make Y evaporate faster at the expense of X evaporating slower.  Obviously entropy must increase, and that's the sum of the areas of the event horizons of X and Y combined, but so long as X gets bigger, I don't know of a fundamental principle which says that Y can't get smaller.
Please, someone disprove this nonsense :)
Edit: Maybe the extra spinning energy of Y makes it heavier by more than the amount that it loses through the Penrose process?
A: There is speculation that a black hole which somehow manages (or is made) to rotate fast enough could turn into a naked singularity. The missing event horizon of such a speculative anomal anomaly would imply that "stuff" could escape from it.
A: Edit: It seems that the answer is no, as far as we can tell presently. 
For relevance, I keep my previous answer, which raised some controversy, below. 
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After searching a little and asking around, I've also come across the Penrose process, and in the comment above Count Iblis mentioned Phantom energy accretion. 
So, all in all, it seems that according to our current knowledge there are only these, rather exotic and slow, processes that could shorten the life of a black hole. 
A: Just FYI, you probably want to wait longer before you just answer your own question like that...
Anyway, black holes can merge thereby forming a new black hole - the two from the beginning no longer exist! Without appealing to some hand wavy argument about how energy is extracted from a black hole, instead black hole binaries lose energy by emitting gravitational radiation. So far 6 black hole binaries have been observed by aLIGO emitting gravitaional radiation as they inspiral and merge, destroying each other to give birth to a new black hole. 
