Instead of sealing off the Abyss, why not just have the deities of your campaign seal off your Prime Material world from the Abyss? That would effectively keep the demons out just the same unless they were already in the world or brought in from a different plane. It would be much easier to seal off one world rather than an infinite number of planes.
This could have some side effect like making Baatezu or Yugoloths easier to summon, or cause them to be summoned inadvertantly--"Hey, I swear I tried to summon Blink Dogs, not Hellhounds!"
Or if you seal off the Astral (one plane), then all Outer Planes are effectively sealed off as well, but then the souls of the dead will have nowhere to go on that Prime world. That could open up a whole new can of worms as well.
What about having your PCs discover somebody shutting down the portals leading into the Abyss? If the demons can't get back home, that's going to create as much of a problem as not letting them out in the first place. Sealing off planes may be impossible, but if all or most of the portals to a plane are destroyed or deactivated, that is almost the same thing.
This could have some side effect like making Baatezu or Yugoloths easier to summon, or cause them to be summoned inadvertantly--"Hey, I swear I tried to summon Blink Dogs, not Hellhounds!"
Or if you seal off the Astral (one plane), then all Outer Planes are effectively sealed off as well, but then the souls of the dead will have nowhere to go on that Prime world. That could open up a whole new can of worms as well.
What about having your PCs discover somebody shutting down the portals leading into the Abyss? If the demons can't get back home, that's going to create as much of a problem as not letting them out in the first place. Sealing off planes may be impossible, but if all or most of the portals to a plane are destroyed or deactivated, that is almost the same thing.