How do I stop the last item on a row stretching to fit the width?

That can be done but will require a code change. You’ll need to add a snippet of CSS to the theme.

Please see this article for instructions on adding custom CSS…

How to add custom CSS

And here’s the CSS you’ll need to add…

.flexbox.flexwrap .flexbox-grid{justify-content:space-around;}
.flexbox.flexwrap .flexbox-grid>li{flex-grow:0;}