I’m sure I remember vaguely if a person blocked ads - you weren’t allowed to force them to see them.
That depends what you mean by "force them".
@Lammert correctly points out that it is your website and you can show what you want on it. That is the user makes a request to your server and you send back a response that includes content and ads. Once received the user has the right to do what he/she likes with the content received, display it in the browser, or block some content such that it is not seen. If you introduce code into your website that programatically forces the browser to display ads then that may be flagged as malware, and could be a "illegal" in some jurisdictions.
But, you can "force" the user into seeing ads by tricking their ad blocking script. You have no obligation to declare ads (in your code) as "ad" or have script called "show ads". Typically ad blockers use such naive clues to identify what is or isn't ads, they also look for requests to known ad serving/tracking domains, like to Adsense or whatever, so you could potentially proxy those request to an obfuscating endpoint, or really do whatever imaginative thing to trick the ad-blocking scripts, and that would not be "illegal".
But all that would require a considerable amount of effort and cost. Again as @Lammert points out:
visitors intentionally blocking ads are probably not going to click on the ads anyway
So you'll spend time and resource for very little return.
You could likely ad some kind of background ad or image that is revealed when no ad is loaded. It would likely require little effort but I doubt it would be effective.
The best strategy would be to add a pop-up when an ad-blocker is detected, asking the user to whitelist your website in their ad-blocker. Personally I hate it when these pop-ups are shown immediately on page load, but you could delay it for some time, giving the user the opportunity to consume some content. Then they could decide "yes this good stuff I wouldn't mind seeing ads in exchange for the value provided" or "no this site sucks, ads will only make it worse". The user that accepts the ads is more likely to click on those ads.
