The advise I’m about to give you is something I’ve figured out my own and seems to work, but I do not know why, if it is necessary, or if there is a better way to manage this problem. If you have a better recommendation, please add it to the comments!
Some web sites can’t be safely closed. If you visit such a site and try to close the browser or tab, a dialog box pops up asking if you really want to close the page, or some such nonsense. It seems to be the case that if you click on that box, only bad things happen and the web page does not go away. The only thing you can do is crash your browser, but if you do that, your browser, being both over-engineered and stupid (as most browsers are) will helpfully open the site back up again when is restarted.
I do not have any third party anti-malware applications installed on my system, as I regard any such software to be malware itself, and I avoid using operating systems that are so vulnerable to attack as to need such overhead-hogging dangerous code running all the time. Perhaps you have such anti-malware and it takes care of this problem for you. If not, read on.
Here’s what I do: 1) close all windows and browser instances except the offending tab. 2) Open a terminal. 3) Use a cli-command to kill the browser. For me, on Linux, using a current version of Firefox, what works is “killall firefox-bin”; 3) click the up-history arrow on your cli to bring the command back to the command line; 4) re-open the browser; 5) As the browser is opening, re-issue the kill command. 6) Repeat two or three times.
Normally, the browser, or at least, Firefox and related browsers with default configurations, will try to reopen the offending web site when it first comes back up, but after “crashing” a couple/few times (and you are crashing it with the kill or killall command) the browser should come up with a “mea culpa” screen asking if you want to restore the previous session or not. Say no. The offending website is gone.
This procedure will have to be adjusted for your operating system, but it works fine in Linux.
There are all kinds of reasons this may not work for you, or may not be necessary. If so, indicate what those are please.