I fixed the sentence.

Daijs gives 4 senses. [2] and [3] seem to be squished together here.
１ 封をする。「小包をしっかり―・ずる」
２ 出入り口などを閉じてふさぐ。「空港を―・ずる」「敵の退路を―・ずる」
３ 自由な発言・行動ができないようにする。「口を―・ずる」「反対派の動きを―・ずる」
４ 神仏の通力などによって閉じ込める。「お札で虫を―・ずる」

We have a tatoeba sentence attached to [1] that maybe(?) belongs to [2]:
"They bribed the witness into silence."

daijs seems to suggest a 3rd meaning, close to "to seal off a building" (ie. "to lock down"). It's harder to find concrete examples of this in the wild.

[1] can be "a letter", "a package", "an envelope", etc. so I added "e.g.". 

Note: 封ずる should be kept identical for both senses. (I'm not going to bother with 口を封ずる, but that is also legitimate. Maybe it should be done...)

@@ -17 +17 @@
-<gloss>to seal (letter)</gloss>
+<gloss>to seal (e.g. a letter)</gloss>
@@ -22 +22,3 @@
-<gloss>to prevent</gloss>
+<xref type="see" seq="2858863">口を封じる</xref>
+<xref type="see" seq="2858863">口を封じる</xref>
+<gloss>to prevent (e.g. movement)</gloss>
@@ -24 +26,2 @@
-<gloss>to block</gloss>
+<gloss>to block (e.g. speech)</gloss>
+<gloss>to seal off (e.g. a building)</gloss>

Ichidan form of 封ずる

@@ -15,1 +15,1 @@
-<pos>&v5r;</pos>
+<pos>&v1;</pos>
@@ -20,1 +20,1 @@
-<pos>&v5r;</pos>
+<pos>&v1;</pos>