Well, you could crash chill it to shut down fermentation, but it'll wake back up once the beer comes to ambient temp. If you're kegging and plugging it into a fridge/keezer to condition, that would work for you (but obviously would take up some real estate).
If you want to keep your FG a bit higher than usual, I'd recommend adjusting your mash temperature rather than trying to fiddle with the yeast during fermentation. Add a few degrees during the mash and your non-fermentables will increase. Keep in mind that a barleywine will probably have a more varied grain bill than a typical batch, which will also result in reduced efficiency (generally). Depending on your previous batches, this may be a variable that you haven't worked with as much.