How do you control the sucker shoots from a white poplar tree?

It's really difficult. White poplars are a very invasive species and they will sucker even when they haven't been disturbed. If you are able to kill the tree (i.e. it's on your property) you will have to soak the stump with herbicide after cutting it down or the remaining roots will send up 100s, even 1000s of suckers.

If the tree is on a neighbour's property, you can resign yourself to endless weeding, particularly in spring. Soaking any root ends you pull up in vegetable oil may help. Spraying suckers with a hot solution of vinegar, dishwashing soap and salt may help. Mulching over the area deeply might control it for a while. Unfortunately, though, white poplar is a really difficult species to manage in urban areas.

Good luck!