That's not possible for several reasons: it's not possible to search for members with less than a certain number of posts, and mass member deletion also isn't available.
Please note that you should be aware of two things:
a) This was requested specifically for my site, for me, and I can't vouch for the code, or how it runs on your site. It's an unofficial add-on and you should treat it as such.
b) Backup your database and other data prior to running it, obviously.