Intentionally knocking over ANY trash can anywhere on the field is prohibited, whether it’s your own or an opponent’s.
Moving your own trash can to your own dumper is legal and there is no rule against the HP tilting the can in it with the dumper to make it difficult for opponents to score bonus balls into it; the opening is not being physically blocked, just less accessible (much the same as when you keep the trash can opening away from an opponent by spinning your robot to stay between your opponent and your trash can.)