Well fibrous taint follows the same placement rules as vines, except it can be on the floor and ceiling. Fibrous taint cannot occupy the same block space as another block. It can also only go on opaque blocks, like stone or dirt. Glass should get taint on it, but a carpenter's block will if it's the full version with an opaque block inside. It can also spread on the flush side of a slab, meaning if it is upside-down, it will spread on the top of it. Fences are probably the best idea when it comes to blocking out fibrous taint. Though it won't stop the swarms completely, it should stop the taint fibers.Fibrous taint can't spread along surfaces with halfblocks/partial blocks on them. You can use ladders, torches, carpenter's slopes, and fences to prevent fibrous taint from spreading back into a cleared area. I haven't tried it yet, but this should also work with carpenter's blocks, trapdoors, wood/stone slabs, and cobwebs (you'll need enchanted shears or a boline to harvest those, though.) Tainted mobs can bypass some of these measures, and will spread taint into the cleared area, but if you can keep them out (fences, again!) it is at least *possible* to clear an area and keep it clear-ish.
Also, keep in mind that tainted soil spreads fibrous taint as well. It's basically soul infused with taint fibers. Also, tainted nodes will spawn taint fibers at any level. So break those in a captured area.