I am free to take a copyrighted book, and tear out pages. But I can't distribute that book. The results of the adblocker are not distributed.
If you define the work as the source code you're not actually modifying it. You're just declining to download subsequent works (iframes, flash, whatever).
(not a lawyer)
And this broad "distributed or used in any other way" doesn't seem very valid either.
Isn't it legal to write with a pen on your book copy if you're not the author?
Creating derivative work is only illegal if you do it without permission from the copyright owner of the original work.
I don't know what you mean by "doesn't seem very valid". It is the law [1].
Annotations normally create a derivative work. That being said, annotations often end up being covered by fair usage (fair dealing in the UK).
[1] 17 U.S. Code § 106 (2) https://www.law.cornell.edu/uscode/text/17/106
Apparently it's not the law, as the link doesn't contain the words "distributed or used in any other way" or anything to the effect of "any other way".
Plus, the "de minimis" thing and fair use exceptions mentioned in your second comment, already scale down the absolute "any other way" qualifier.
Maybe "used in any other way" confuses you? It's standard legal language and it means publishing, public performance, creating mechanical copies, etc. The possible uses of a copyrighted work are numerous and due to advances in technology that list grows constantly. This expression makes it unnecessary to list every known possible use, or yet-to-be-discovered futures uses.
The fact that "certain exceptions" weren't mentioned in the first comment. I quote: "the copyright owner of the original work has the exclusive right to prepare derivative works, regardless of whether the derivative work is published, distributed or used in any other way."
There was no reference to "certain exceptions", "fair use" and the possibility of "minimal" (and thus allowed) changes.
That, plus the use of "used in any other way" (a "standard legal language" as you say) as part of a casual language comment, left the impression to the reader that only the copyright owner or someone with permission from them can create derivative works, period.
So while this is cleared out now after the extra explanations, the initial comment was quite unclear.
Does not follow. Ad block is set up by the user to block connections. Your work doesn't change; the user just doesn't see the full work. Kinda if I gave you glasses that blocked the color red and then you went and looked at an art gallery wearing them. The art hasn't changed, but the item I gave you, which you willingly wore, just stops some part of the art from being displaying into your eye.