Would that be a violation of some implied terms of agreement under which the page host is offering the content?
It can’t be illegal, as no agreement will be in force as you hit the page. However, if they can determine that you are not displaying the banner ad (via cookies, mostly), they have the right to not return any content at all (or pop up a message, or replace all images with inverted versions, or some other annoyance).
There could be trouble if its an internet cafe or public hotspot wifi thing being filtered. The key is the public nature…
eg what if you are using wifi at McDonalds and they censor KFC / Hungry Jacks /Burger King/Pizza Hut adverts ?
The producer of the content (web page or tv channel, dvd, etc) can claim that a modified version is a breach of their copyright, they control what is released.
It’s not illegal that I’m aware of, and you didn’t agree to any terms when you viewed the site. (Well, it’s possible you did I suppose, but I’m sure you’d remember that.)
This is assuming when you say “deleting” you mean “removing from your client” and not “hacking into their web server and deleting the ads for everybody who views the site”. Hacking into a web server is illegal almost everywhere.
I do think ad-blocking is kind of a dick move if you’re enjoying the otherwise free content of the site.
It’s not free if they demand you view ads with it. It’s more of an unspoken negotiation: “I’ll show you my content if you listen to my sponsors.” My counter-offer, and the default of AdBlock Plus these days: “I’ll look at your sponsors’ messages as long as they’re not too annoying.”
Websites that get it change their ads to be less intrusive. Websites that don’t, yet continue to offer their content, either have enough money from other users that they don’t care, or soon fold. No big loss either way.
You should be OK as long as you don’t remove your mattress tags.