Important to note that if the creature in question is summoned (rather than called), then it's a generic, average, non-unique individual member of that type, unless your game uses the Summoning Individual Monsters optional rule.
So with a summoned demon, when it dies it just goes poof, discorporates and reforms, ready for use some time later. If the demon in question has been called, then when it dies that's all she wrote - it's plain ol' regular style dead.
As for the Plane Shift and come right back part - check out the spell description. Plane Shift isn't very accurate as for where you end up on the target plane: "Precise accuracy as to a particular arrival location on the intended plane is nigh impossible. From the Material Plane, you can reach any other plane, though you appear 5 to 500 miles (5d%) from your intended destination." So at that point any outsider looking to pop right back into the thick of things would not only have to PS back to the Prime, but then probably Scry on the person they want to get at, then Teleport there before any havoc wreaking can begin.
And frankly, any outsider - demon or otherwise - that can bust out with those kind of resources at the drop of a hat deserves to be able to be very hard to be rid of. But this would be more for dealing with just banishing said outsider and assuming they're dealt with, rather than killing them.
If you wanted some badass demons that you can't kill normally, and otherwise killing them just sends them home for a while to stew, maybe look into cooking up some demonic equivalent of a lich's phylacrity. Takes care of some things rather handily, like not every rank and file demon being that invulnerable (unkillable lemures? bleh), killing them just sending home (body reforms at phylacrity, which is in hell somewhere), and having to kill them on their home plane in order to keep them down for good (gotta destroy their phylacrity, which coincidentally is back home).
