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 <dict> block:
<key>DictationOnSpaceKey</key>
<false/>
Update a different file
Using iFile, edit the file /Users/Library/ConfigurationProfiles/UserSettings.plist
Set restrictedBool > allowDictation > value to Off or false. It should look like this:
<key>allowDictation</key>
<dict>
<key>value</key>
<false/>
</dict>
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.