If these are suckers, then a systemic herbicide is probably needed to control them effectively. If they are seedlings, then mowing alone might be sufficient while they are still young. If you can't tell (and our guess would be suckers, especially if someone in the immediate area cut down a mature Tree of Heaven in the past couple of years), try pulling some up. Seedlings will probably pull up more easily with roots attached, whereas suckers can pull up with more force and/or will generally snap off without any roots when they detach.

Miri