Hi stetwood,
Welcome to Laptop GPS World.
After you mistakenly start searching POI's along your entire long route, try to just press Esc-key, and then just wait. It may take a long time, but it should eventually cancel the search.
I just tested it with a route from Toronto, Canada to Mexico City, Mexico. It is over 4,000 km or 2.500 miles long route, and I set the search to the maximum radius of 80 km / 50 miles around the whole route. I'm not even going to guess how long the search would take. 30 minutes? An hour? Hours? But I just simply pressed the Esc key on my keyboard, and within about a minute the search stopped.
Would I wish the search stopped immediatelly? Yes. But knowing that the cancel feature does work, I will gladly wait that one minute.