In Slovakia it does have legal power. The advertising law prohibits unsolicited advertisment by paper mail, e-mail and telephone. For paper mail it's opt-out (No junk mail sticker). For email and phone it's opt-in (no cold calls). Works well for paper mail, but not the others.

Also here it is not the postal service that distributes the junk mail leaflets but independent agencies. The Post has monopoly on letters by law. Other deliveries have theri commercial providers alongside the state Post.