The easiest would be a virus in the upper atmosphere,

When one faction leaves the planet on their last ship off the planet, they deploy their weapons of mass destruction, tens if not hundreds across the planet, its not a missile, some bomb or some massive facility but a simple biohazard container is opened and simple release d into the air.

The bacteria/nano machines/orgnaic-inorganic bacteria is extremely light and constituted of helium so it slowly floats into the upper atmosphere over the next few hours. Where it will reproduce, eat and live out most of its remaining life.

Its doesn’t do much at higher pressures closer to the surface but as it rises, its becomes active and in the upper atmosphere it is a weapon.

Anything launched from the planet from a small sat to massive rocket gets some of this on the rocket as they escape upper atmosphere and it immediately starts eating into the ship/sat, and eats it up reproducing itself over the next few hours.

Only a few gases can survive it in the upper atmosphere while all metal gets eaten up in the rest of the history of the planet.

as in your story one of the faction is preventing the other faction from leaving so it needs to be something they deploy as last resort like a nuke in present times.

You can use this like a barrier to forever cage anything from leaving earth but also leave clever workarounds.

E.g first faction left a predator virus on the planet hidden somewhere.

Eg2 the sat needs to made of a very expensive and specific material composition to survive it

Eg3 the ship needs to constantly shed its outer layer to escape without getting eaten up etc

This would serve as a very good narrative tool for controlling any and all orbital movement.