Before you read further, remember: Subverting a Free Webspace Provider's popups or other gratuitious adverspamming may be against the TOS, and as such, they reserve the right to boot your ass out and delete your page if they catch you doing it! While many are thick-headed enough not to waste their precious brain-time worrying about it, this is a public page, so even the FWPs themselves can (and a couple do!) read up on every method detailed below. Consider these a "temporary" fix to keep an FWPs popup from annoying your visitors while you redirect them to your site/wiki/blog/forum's NEW location (when a greedy FWP puts that kind of shit on your page, you move out of course!) In other words, you should be putting this on your replacement "My page has moved!" signpost pages, not your actual webpage.
These bastards. I was snooping around in the source code, for no particular reason, and found that they had inserted a Matomo tracking script into every single wiki page. However, we can use the copyright/Creative-Commons notice displayed on most pages, which is verbatim HTML code, to disable most occurences of this script. Insert the following at the bottom of your wiki's copyright notice, which should be at yourwiki.miraheze.org/wiki/MediaWiki:Copyright.
<ul id="footer-places">
<li id="footer-places-privacy"><a href="https://meta.miraheze.org/wiki/Special:MyLanguage/Privacy_Policy" class="extiw" title="m:Special:MyLanguage/Privacy Policy">Privacy policy</a></li>
<li id="footer-places-about"><a href="/wiki/Project:About" title="Project:About">About {{SITENAME}}</a></li>
<li id="footer-places-disclaimer"><a href="/wiki/Project:General_disclaimer" title="Project:General disclaimer">Disclaimers</a></li>
<li id="footer-places-termsofservice"><a href="https://meta.miraheze.org/wiki/Special:MyLanguage/Terms_of_Use">Terms of Use</a></li>
<li id="footer-places-donate"><a href="https://meta.miraheze.org/wiki/Special:MyLanguage/Donate">Donate to Miraheze</a></li>
</ul>
<ul id="footer-icons" class="noprint">
<li id="footer-poweredbyico"><a href="https://meta.miraheze.org/wiki/Special:MyLanguage/Miraheze" target=""><img src="https://static.miraheze.org/commonswiki/f/ff/Powered_by_Miraheze.svg" alt="Hosted by Miraheze" width="88" height="31" loading="lazy"></a><a href="https://www.mediawiki.org/" target=""><img src="/w/resources/assets/poweredby_mediawiki_88x31.png" alt="Powered by MediaWiki" srcset="/w/resources/assets/poweredby_mediawiki_132x47.png 1.5x, /w/resources/assets/poweredby_mediawiki_176x62.png 2x" width="88" height="31" loading="lazy"></a></li>
<li id="footer-copyrightico"><a href="https://creativecommons.org/licenses/by-sa/4.0/" target=""><img src="https://meta.miraheze.org/w/resources/assets/licenses/cc-by-sa.png" alt="Creative Commons Attribution-ShareAlike 4.0 International (CC BY-SA 4.0)" width="88" height="31" loading="lazy"></a></li>
</ul>
</footer>
<noscript><!--This emulates the appearance of the links and licensing icons that are usually at the bottom of the page.
Inspired by a similar page from a long-gone era on Cexx.org.