This is overkill. If you are asuming adblockers will remove anything with the id "adTeaser" (as per the code), just use that to display the message! No need for the "setTimeout" or even Javascript:
<div id="adTeaser" style="position: absolute; z-index: 10000; ...">
Please turn on your <a href="https://github.com/gorhill/uBlock#ublock-origin">ad blocker</a> to continue browsing this site
</div>
Sure, my mistake. Flip that "position:absolute" for "position:fixed". Note how I wrote "z-index:..." to imply an element covering the whole page.
Also note that activating your adblock does not disable the message in the OP, you'd also need to refresh the page since you are effectively removing the body.