Just like anyone else.
No, wait, hear me out, it's serious!
In Pathfinder, unless a creature is actually being summoned, in which it is not a real creature but a magical copy of them (depending on your setting), it can be killed permanently like any other creature. This is evidenced on the spells that are most likely used to bring them to the material plane, Planar Binding and Planar Ally.
And that is because both spells are from the Conjuration (Calling) school, which says:
Calling: a calling spell transports a creature from another plane to the plane you are on. The spell grants the creature the one-time ability to return to its plane of origin, although the spell may limit the circumstances under which this is possible. Creatures who are called actually die when they are killed; they do not disappear and reform, as do those brought by a summoning spell (see below). The duration of a calling spell is instantaneous, which means that the called creature can’t be dispelled.
You will see no mention of outsiders being unkillable on the rules about binding them.
Unless they are really powerful
So, unless this demon has some sort of Contingency or special ability saying it is sent back before it is killed, it dies. For instance, Demon Lords And Archdevils are mythic creatures, as such they are all immortal as they have 10 mythic ranks and at 9th they gain the Immortality mythic ability.
At 9th tier, if you are killed, you return to life 24 hours later, regardless of the condition of your body or the means by which you were killed. (...)
At 10th tier, you can be killed only by a coup de grace or critical hit made with an artifact.
Demons being unkillable is a thing that is a known trope in older d&d editions, specially sealing powerful demons.
But if this demon, in particular, is unkillable...
If, for whatever reason, mechanical or flavorwise, the creature cannot be killed outside of their home plane, enter the Devil's Key. A redeemed longsword that can permanently slay outsiders by plane shifting both the wielder and the outsider to the outsider's home plane, and once slain, they are permanently destroyed.