Your web server is simply providing data (a bunch of HTML) to anyone who sends an HTTP request for your web page. That's all your server is doing.

It's not telling the client (the browser) "here's a bunch of HTML, but you can only use it if you show ads". There is no contract or agreement to display ads, implied or otherwise.

In the end, all you can do is simply ask the user to display ads. If you want to enforce that the user views ads, the web probably isn't the right medium for your product.