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"?

PS: I posted the same question to Physics but I think it applies well to both SEs. So I hope it is okay to try here as wwell.
It can merge with another black hole, if that counts. A simulation of that is illustrated here

That aside, no black hole with decay more quickly, as far as I can find out, than the "normal" decay of a non-rotating, uncharged black hole into which nothing is falling. Let's call this the "standard rate of decay".

"Feeding it" (allowing any matter or energy to fall into it), will reduce the decay below the standard rate, or even reverse it. Since the CMB pervades space and will fall into the black hole, blocking this (for instance by surrounding the hole by an opaque refrigerated shell) will be necessary to allow it to decay at the standard rate. Indeed for a black hole of mass over roughly $10^{22} kg$ the CMB would feed it faster than it decays, if not blocked.

Energy from the spin of the  black hole can be extracted using the Penrose process. Since this answer shows that rotation also reduces the emission of Hawking radiation, you can clearly speed up the decay by extracting energy from as much of the spin as possible, but the fastest decay you get is, once again, the standard rate.