Once a black hole is formed, is there anything other than Hawking radiation which shortens its life?

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.