The correct treatment is to dig down to where they are sprouting from (if beneath the soil) and cut it off flush.
Problem is, they seem to keep sprouting!!!!

I've been successful with a persistent sucker problem on a grafted pear by cutting the sucker back short enough to jam a tin can over the top and into the soil to exclude all light. It worked in this case but don't ask why - it's dark underground anyway but perhaps once the growing tip has discovered light it can't live without it.

Roundup "painted" directly on the fresh cut will work. Follow label for concentration mix. If the sprout is small in relation to the parent plant then (the cross section being small) there won't be enough Roundup to kill the parent but you may notice some effect. If the sprout is large, or the parent plant small, there could be a killing of both.