I don't think you can get at their resource surplus. But you can put their tax rate at the highest level so they won't build up much more of one, and I don't think sectors actually have a resource cap.
Sectors can hold the same maximum of resources as you can, that's why you'll see they cap out at 11,000 energy or so in the late game.
That said, I've found almost all of the problems people seem to have with sectors not building stuff is because the sector is far, far too big. Like one mega sector big. As far as I've been able to deduce, the more stuff there is in a sector, the more funds it will want to sit on to ensure it doesn't accidentally bankrupt itself during a construction splurge after an upgrade is researched or something.
I don't know if I'm totally off base with those observations, but working with a number of smaller sectors seems to have done wonders. As games go on and systems become more fully developed, I then merge my early sectors to make them bigger. It does seem to be a bit of a hidden pitfall if that is true though, because there's no openly advertised cap on how many systems you can stick in a single sector before it becomes ineffective.
As for slavery in sectors, I imagine the caste system should largely fix those concerns. Instead of enslaving on the wrong tiles, I suppose now it'll be an argument of whether the sector is effectively using pop traits on tiles, which to be fair, is more of a quibble than a fundamental issue.
