Controlling Suckers

My Dad’s Goldrush suckers relentlessly. I grafted a couple onto seedling understock to avoid this problem. Is there something I can put on this tree in the meantime to stop or reduce this constant battle.

I have not used this,but I know it was used on street trees years ago to keep them out of power lines.
Also a fruit thinner, so caution is in order
It’s NAA