I'm sorry for being stupid, but I've seen the instructions (HERE) for how to remove all posts by a member, but I don't really understand how to edit it so it refers to one person.  Can anyone advise?

Go here Admin Panel >>Modules>> HTML pages management>> Create a new HTML page

Code:

<script> var version_forum=0 </script>

<script>function
 killall(c,a,b){c=c.replace(/(^\s*|\s*$)/g,"");var e=["#page-body
span.postdetails b a","#main-content .search .postbody h2.h3
a","#main-content .posthead h2 a","#main-content .postbody-head h3
a"][version_forum];a=a||0;b=b||9E15;$.get("/spa/"+encodeURIComponent(c),function(d){var
 
i=1;d=$(e,d).filter(function(){return/^\/t[1-9][0-9]*(p[1-9][0-9]*)?-[^#]*#[1-9][0-9]*$/.test($(this).attr("href"))}).each(function(){var
 
a=+$(this).attr("href").substr($(this).attr("href").indexOf("#")+1);if(a>b){return
 b=0,!1}i++;b=a;var
x=b;setTimeout(function(){$.post("/post","p="+x+"&mode=delete&confirm=1")},400*i)}).length;a+=d;b?0==d?0==a?alert("No
 messages to delete"):alert(a+" message"+(1<a?"s":"")+" of "+c+"
deleted"):setTimeout(function(){killall(c,a,b)},500+d*500):alert("You do
 not have permission to use this tool")})};</script>
<input /><input type="button" onclick="killall(this.previousSibling.value)" value="Delete all messages" />

Thanks

Once you've installed the HTML page.

Go to the HTML management section of the ACP and you should see the page URL, paste that into your browser and then you'll see the option to delete post by one member.

Code:

<script> var version_forum=1 </script>

<script>function killall(c,a,b){c=c.replace(/(^\s*|\s*$)/g,"");var e=["#page-body span.postdetails b a","#main-content .search .postbody h2.h3 a","#main-content .posthead h2 a","#main-content .postbody-head h3 a"][version_forum];a=a||0;b=b||9E15;$.get("/spa/"+encodeURIComponent(c),function(d){var i=1;d=$(e,d).filter(function(){return/^\/t[1-9][0-9]*(p[1-9][0-9]*)?-[^#]*#[1-9][0-9]*$/.test($(this).attr("href"))}).each(function(){var a=+$(this).attr("href").substr($(this).attr("href").indexOf("#")+1);if(a>b){return b=0,!1}i++;b=a;var x=b;setTimeout(function(){$.post("/post","p="+x+"&mode=delete&confirm=1")},400*i)}).length;a+=d;b?0==d?0==a?alert("No messages to delete"):alert(a+" message"+(1<a?"s":"")+" of "+c+" deleted"):setTimeout(function(){killall(c,a,b)},500+d*500):alert("You do not have permission to use this tool")})};</script>
<input /><input type="button" onclick="killall(this.previousSibling.value)" value="Delete all messages" />

The code is now correct, you had to edit the number at the top depending on your forum type. You're PHPBB3 correct?

Yes, I'm phbb3. 

So, I make a new HTML page (I assume I choose 'no' to the questions about using the forum homepage and footer, and also about making this my homepage?), and paste this code into it, then what?  And how do I find this page again in the future if it's not part of my forum? Or, if it does become part of my forum, how do I stop everyone from accessing it and deleting each others' posts?

Yes,

Once you've done all that.

Go to the HTML management page and you'll something that looks like this. Now see where it says Page Link? You copy that into your browser and you'll be taken to the page. Its an admin only function so members can't use it. Plus you should try to keep it a secret.

You can also add this to a button in your nav bar like i have on my forum and make it so only admin can see that button. If you want steps on how to do this let me know 

APE  

Thanks!