There are two aspects to tainted lands that are important:
Principally there is the tainted lands biome - this is actually quite innocuous - it does not spread by itself, mostly acts like a magical biome - it does however allow fibrous taint to grow and will corrupt (taint) any nodes that occur in it.
fibrous taint - grows over the surface of solid blocks in tainted lands biomes. fibrous taint is NOT spontaneously created in tainted lands biome - it must spread from existing fibrous taint. (tainted dirt and crusted taint blocks act like blocks with fibrous taint), The only things that create new fibrous taint are - tainted sheep grazing - tainted creepers exploding, tainted nodes, extremely large quantities of flux goo, and the primal focus. And, of course, bottled taint.
If you manually remove all fibrous taint (including any crusted taint or tainted dirt blocks) and none of the other things are present, fibrous taint will stop spreading / spawning.
Fibrous taint cannot survive outside tainted lands biome but when it reaches the edge it will - extremely slowley - grow the tainted lands biome.
It also cannot spread over non solid blocks like fences, glass, torches etc.
Fibrous taint that covers 3 sides of dirt and wood blocks will convert those blocks to tainted dirt and encrusted taint blocks.
Fibrous taint can only spread to adjacent blocks horizontally, but can jump up to 2 block vertically - so the roof of a underground facility in a tainted lands biome needs to be 3 block thick (and those blocks can't be dirt or they will (probably) eventually become tainted).
