I consider Madara's immortality to be like Hidan's. He can't be killed but unlike Hidan, he can regenerate as long as there's a part of him there to regenerate from.
He himself isn't the Juubi though so I'd say if he gets erased to the point that there's nothing left of him, there'd be nothing left for him to regenerate from. After a while, the Juubi would respawn which is why Hagoromo decided to seperate its chakra and seal it before his death.
I also believe that the seals were meant for the Juubi since it would keep returning to wreck havoc on the world until it gets sealed. Naruto and Sasuke were only able to meet the Sage when they were at the brink of death. This happened to happen during Madara's reign as a Jinchuuriki since he put them in that state.
Madara is immortal and can regenerate. Just being immortal won't make you omnipotent so even though someone like Hidan can't be killed, you can still "erase" his body. I wonder what happens to them though.
Blowing Madara up to smitherings wouldn't work. He's immortal so that wouldn't kill him and unlike Hidan, he can regenerate. He'd wind up regenerating from whatever's left of him. You'd have to erase him completely but even if you do manage that and Madara's body is erased, after sometime, the Juubi will respawn.
Guy almost did kill Madara since he was being reckless. He had regeneration but apparently wasn't immortal yet. I think I remember a part of his head missing too so I still don't know what it would've took to actually kill him at that point (Maybe if Guy would've destroyed his head and not just a part of it/aimed for the head instead of the chest).
This also makes me question if Obito was immortal since he was the one who seperated the Shinju from his body in the first place.
That's my take on it anyways.
