Robinia Mop Tops Suckers after the trees were removed

[Seasunsail]

I had removed 12 Robinia Mop Tops from my garden about 4 months ago by professional tree removers including the stump ground down – it was an established tree about 20 or so years old. Landscaped the area with other plants & 12 Established Japanese Maples in its place shortly thereafter.
Little did I realise the root system of the Robinia Moptops was busy preparing its resurrection tactic.
I'm noticing more and more little suckers showing up all over the place with massive thorns. The tree removers should have poisoned this tree when they first chopped it down, or even before. They clearly didn't know this was a thing.
Has anyone else had a similar experience and did you have any 'success' in keeping them at bay and eventually killing it? It seems like letting the suckers grow a bit, then then chopping and poisoning them is the best way to slowly get rid of it. But this will likely go on for quite a while I think. Digging down and poisoning the larger roots that came off the stump probably isn't going to do anything at this stage as the entire root system has started to do its own thing so it wouldn't be affected by that I don't think.
What poison and what ratio of dilution is the best at getting to these suckers?
Any advice??

 

[cpp gardener]

Your best bet is fresh cuts (stems or roots) and the strongest undiluted glyphosate (Round-up) you can find. I hear good things about triclopyr too. There will be multiple applications needed for complete control.

 

[Oliver Buckle]

I don't know if this has any basis, but ...
I would would use glyphosate to paint the leaves leaves first, bigger area and possibly easier absorption, then when they wilted cut and paint the fresh cuts