I noticed Adblock Plus injecting some CSS that hides the advertisements, but I think that it does more than that - for example blocking HTTP requests.
How does Adblock Pl
Here it is how it is done: https://adblockplus.org/en/faq_internal#elemhide
They just disable the block the adblock with the list of ids that they have in hand at user agent stylesheet i.e. at your browser level with its JS.