Q.pruning gum tree
I want to train tree stump to be a low height “hedge.” Is a river gum about 15 mtrs before cutting down. Now is re-shooting vigorously, I don’t have heart to grub it out, give it a second chance but keep under control. Best suggestion? Is along front alignment 600mm from boundary. Stump is about 100 mm x 500 mm. Shoots so far are 1500mm x 1500, numerous branches from 2 spots around circumference. Tree grows while you watch it.

Hedge pruning can be done with certain species of Gum Tree.
The article below will help you.
https://www.gardeningknowhow.com/ornamental/trees/eucalyptus/eucalyptus-pruning.htm