People were joking about turning off Skavenblight's Waystone energy by way of the mapping action, but... uh...
Article: The Black Pillar of Commandments, also called the Pillar of Commandments or simply the Black Pillar, is a great pillar of black, obsidian-like stone made from pure Warpstone located in the Skaven Under-Empire's subterranean capital city of Skavenblight.
If you squint, this actually does kind of look like what you'd expect from Skaven leeching off of the Waystone Network with a nexus-analogue?
The problem with that line of reasoning can be summarized thus:
Do we think the Asur somehow did not notice the race of chaos eating sentient rat people with warrens under every city in the Old Wold, including Marianburg?
I do not think that is the case. The idea that they would not know how to turn off stones leading to a nexus that had been tainted is also kind of hard to swallow, so they must have chosen not to do it... for several thousand years. This is not a Vlag situation where the elves would have no way of knowing the stones were put to the use of Chaos, they have significant presence in Tilea. My guess for why the Asur have not turned off Skavenblight if such a thing can be done is that it would unite the underempire in rage and cause a rat apocalypse in Tilea and beyond.