I guess I assumed too much in my post in that I've being seeing a lot of posts about it in my "Matches" subscription and take that as enough background to not get specific. So my mistake if my language somehow led to the conclusion that I was saying ajax is a security issue.
So, the "permissions issue" is being widely discussed not as a security issue, but because a number of people have had their upgrades broken by specific security plugins. The cause of the break is that, in hardening wp-admin, they are blocking access to the ajax php file in the wp-admin directory. The solution being offered is to use a specific htaccess rule to allow all on that file.
My question was simply to establish if ajax functionality is even required. If it is not then the option of disabling it would be one that I, personally, would prefer over the "allow all" option being offered.
I also personally like to minimize the amount of code libraries required for my site to function. I use ajax nothing, except in WP itself. Less code, fewer potential targets, down the road, not today. So again, if it is not critical to WP's operation, if there is a way to turn it off, I'd use it.
Your answer apparently is "No."
So that answers my How-to question -- you can't do it.