I have a mop top robinia in my garden, anyone any info on how to stop or
control it suckering,

If you mean Robinia pseudoacacia or a similar species, this is what they do.
The only way to prevent it is to remove the parent tree. For more valuable
trees, like crabapples, there are hormone sprays you can use, but for a cheap
weedy tree it's not worth the trouble.

Iris,
Central NY, Zone 5a, Sunset Zone 40
"If we see light at the end of the tunnel, It's the light of the oncoming
train."
Robert Lowell (1917-1977)