There are four things that you can try:
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
Update a different file
Using iFile, edit the file /Users/Library/ConfigurationProfiles/UserSettings.plist
restrictedBool > allowDictation > value to Off or false. It should look like this:
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.