How to disable voice dictation on an iPhone 4S running iOS 5.1 or later?
There are four things that you can try:

Disable Siri

You can disable Dictation by disabling Siri. Navigate to Settings > General > Siri and turn Siri "Off".

Update PList File

However, understandably, you may want to use Siri, and since you have a jailbroken phone, this appears possible by editing a .plist.

As per this MacRumors post, edit the file /User/Library/Preferences/com.apple.keyboard.plist. Edit it to add the following lines inside of the &lt;dict&gt; block:

&lt;key&gt;DictationOnSpaceKey&lt;/key&gt;
&lt;false/&gt;


Update a different file

Using iFile, edit the file /Users/Library/ConfigurationProfiles/UserSettings.plist

Set restrictedBool &gt; allowDictation &gt; value to Off or false. It should look like this:

&lt;key&gt;allowDictation&lt;/key&gt;
&lt;dict&gt;
  &lt;key&gt;value&lt;/key&gt;
  &lt;false/&gt;
&lt;/dict&gt;


Install "NoDictation"

NoDictation is a free jailbreak tweak available from the BigBoss repos (available by default on most iOS jailbreaks). If neither of the previous options work, it may.