to seal (letter)
to prevent; to forbid; to block
「封ずる」 has another reading with different meanings.
大切な書類を厳重に封じた。
I carefully sealed the important documents.
蝋で封じて送ってください。
Please seal it with wax and send it.
この手紙は開封されずに封じられたままだった。
This letter remained sealed without being opened.