How do I open or smash these silver blocks and padlocks?

In order of first available in terms of game process, the following can be used to destroy these blocks:

Berserker Orc character

Purchased at a cost of 200000 studs.

Becomes available after completing the story level Helm's Deep

The can be found on the world map, near Helm's Deep.
You can find this character behind the Deeping Wall.

Mithril Fireworks item

Forged with a cost of 16 Mithril blocks.

Plan becomes available after the storyline level The Black Gate.

The can be found on the world map, near Barad-dúr (Sauron's Tower).
To plans are above the door of the gate on the east side.