seal off

Also, seal up. Close tightly or barricade to prevent entry or exit. For example, We're sealing off the unused wing of the building, or The jar is tightly sealed up. Dating from the first half of the 1900s, this idiom uses seal in the sense of "close securely," as one used to do with a seal of wax.

The verb has 2 meanings:

Meaning #1: make tight; secure against leakage
  Synonym: seal

Meaning #2: impose a blockade on
  Synonym: blockade