What I've done is to tune a sine wave to match the frequency of the tinnitus, then notch that frequency in white noise, and blast the result for a few minutes. My tinnitus has not been permanent, it's just the rare occasion where I forget earplugs for a concert, and come home work ringing in an ear.

This technique makes the ringing go away for a while, but it creeps up again after some time. Thankfully, it has always completely gone away after a day or two.