The best way is to just remove them as they appear. Sorry, I do not know of any long term solutions to this.

I tend to doubt it. Some say nitrogen fertilizer stimulates trees to sucker more but I haven't fertilized apple trees for many years and suckering is ongoing. Some rootstocks are much worse than others but I guess we're out of luck there.

There is a product called Sucker-Stopper manufactured by Monterey that is a synthesized plant growth hormone (NNA) intended to reduce/retard the development of tree root suckers. It is essentially the same product that commercial orchardists use.

The commercial product was called Tree Hold. Many years ago I sprayed this around apples on M-7 rootstock at an orchard I worked at just after getting out of college. It would burn the suckers to the ground and really did stop them for the year. Although they would grow back the following year. So it was a yearly battle but I was impressed with how it worked. It was sold in gallons and had to be diluted according to the directions. If you don't have many trees, the Sucker-Stopper mentioned above by gardengal may be a cheaper way to go assuming it is packaged for home use and for those who do not need to make gallons of the stuff.

I wack them off with a brushwacker. When trees get older you'll have less.

I agree on young ones. I just cleaned up a neglected 30 yr old orchard with one to three inch suckers. The re-gen one these large one will be tremendous. I was hoping to stop it before it happens. The health of the tops is poor from neglect, weather, zone3,maybe zone 2 , and old age. Maybe m7.

New growths is easy to remove with wacker.