Oh sorry, you're not aware of the context. That first line was specifically referring to the corruption of Waystones, not Warpstone in general. This is the paragraph preceding it:
"Leylines are also a very attractive prospect for those that draw upon mixes of the colours of magic, either in the clean form of High Magic or in the dangerous form of Dark Magic, because all colours are present as they flow along the Leylines. Certain powerful and entirely corrupted warlocks and necromancers have found ways to corrupt the Leylines as they pass through Henges. If the exit path of magic entering the Henge can be blocked or destroyed, then the magic flowing into the Henge will be trapped within the stone circle itself, unable to move onward and unable to leak out into the atmosphere unless tapped into directly by a magic user. Such a thing is terrible, for not only does it risk destabilising the delicate balance of the Great Vortex, but it also means that the magic contained within the Henge will gradually combine and stagnate into that most dangerous of all Aethyric energy, Dhar."
That paragraph was specifically addressing the acccumulated Dhar within a leyline. If there is no block, then any Dhar that is generated is usually cleared up eventually when the leyline does its job.