There are commercial products available that will kill suckers, available on line and possibly at menards and the like. Google "sucker stopper" will provide a list of products, though I never personally have had to use said products so I'm not certain of their efficacy. Do not use a herbicide application as this will affect the roots which would kill the trees.

https://hortnews.extension.iastate.edu/managing-suckers-fruit-ornamental-and-shade-trees

It is very common for poplar to spread using suckers however, and there is no good one time control to keep this from happening. Best method for seasonal control is to cut them down at the ground level or just constantly mowing them through the season, but they will continue to sprout each year - particularly in the spring when the roots are sending energy into growth for the season. During the summer and fall sucker growth is usually much reduced.