This tree is prone to producing dense thickets of suckers both stem and root. It has finally become root bound in the pot and this has stimulated sucker growth, in this case, on the stem. Cuttings suckers close to the stem is about all you can do but from now on you will not be able to stop this process since this is what the black locust likes to do. This tree is really not suitable for small gardens or potting. Unless you have a large property I would not consider replanting this invasive tree in the ground. It has given you 7 years of good growth but now it might be time for it to go. This tree is often sold as a potted ornamental tree which I think has not been a good thing to do due to its bad growth habit.

Thank you, much appreciated

As Loosestrife says, it's time for it to go. This is a tree that can easily grow to over 100 ft in the right conditions and a pot is not the right condition. It is in distress! If you don't have space to plant it in the ground then the kindest thing is to give it to a friend who can give it a good home. As a last resort, it makes good firewood.