Let's take another, more similar analogy: some secure messaging programs are designed to resist network analysis, which invariably involves sending some kind of "noise" to clients whom you are not actively communicating with, to mask your actual communication. Depending on your point of view, you could frame that as an "attack on your ISP" - after all, aren't you sending unnecessary junk traffic? This is the same thing, except for the web instead of messaging.
In general though, not picking on you in particular: I really see a lot of nerdfolk who don't understand that intent and context do matter. I see it particularly often in legal contexts where people think they've figured out one weird trick to escape the law. Like, I don't know, people who think that they can claim they forgot the password to an encrypted system they use on a daily basis, and the judge is just a computer who will say "oh no I am defeated, beep boop", instead of a human being who will think about whether that's reasonable and throw them in jail for contempt.
It would be wildly unreasonable to think that AdNausium is a click-farming scheme, or a DDOS attempt, or anything of the like. Context does matter.
Also, if some of your clients actually using your primary "features" (clicking your ads after they load a page) is a problem for you, it's hardly their fault. By all means feel free to discontinue serving them ads if you don't like it.
Blocking users who run AdNauseum does seem entirely appropriate to me. Blocking the plugin from Chrome also seems appropriate to me.
I don't see a lot of merit in AdNauseum. It feels to me like a petty "fuck you" response from people who don't like ads.