Can the dictionary be turned off?
ReaderPen
If the ReaderPen firmware version is 7.0.89 or newer there is a setting called Exam Lock that disables Scan to File, Dictionary and Recorder and leaves only Text Reader active.
If the firmware is older it is possible to update. Check here for details.
LingoPen
LingoPen firmware supports a function called Exam Lock that disables Scan to File, Dictionary and Recorder and leaves only Text Reader active.
ReaderPen Secure
For ReaderPen Secure it is not possible to turn off dictionaries.
DictionaryPen
For DictionaryPen it is not possible to turn off dictionaries.