A steamroller is a large, heavy vehicle with wide, solid metal wheels, which is used to make the surface of a road flat. In the past steamrollers were powered by steam
If you steamroller someone who disagrees with you or opposes you, you defeat them or you force them to do what you want by using your power or by putting a lot of pressure on them. They could simply steamroller all opposition. to make sure something happens by using all your power and influence, or to defeat your opponents badly