When Rose Tyler possessed the Time Vortex, she brought Jack Harkness back to life — and because she didn't have control over it, Jack Harkness became immortal.
From The Sound of Drums episode transcript:
JACK [OC]: Last thing I remember, back when I was mortal, I was facing three Daleks. Death by extermination. And then I came back to life.
JACK: What happened?
DOCTOR [behind door]: Rose.
JACK [OC]: I thought you'd sent her back home.
DOCTOR [OC]: She came back. Opened the heart of the Tardis and absorbed the time vortex itself.
JACK [OC]: What does that mean, exactly? (Yana is crying.)
DOCTOR [OC]: No one's ever mean to have that power.
DOCTOR [behind door]: If a Time Lord did that, he'd become a god. A vengeful god. But she was human.
(Flashback to Parting of the Ways, Floor 500 of the satellite.)
ROSE: I bring life.
(Jack remembers his first coming back to life.)
DOCTOR [behind door]: Everything she did was so human. She brought you back to life but she couldn't control it. She brought you back forever. That's something, I suppose. The final act of the Time War was life.
But, she also created "Bad Wolf" signs across time and space. Are those signs also indestructible just like Jack Harkness? Is there a reference to back this up?