There is no setting to manually turn off the internal mic. Each app enables it as needed.
A properly seated microphone plug should disable the iPad's internal mic. At least it does with my standard Apple headphone jacks, for those apps I've tested. That's not a huge number by any means.
Here is a picture of the Apple plug (it's industry standard, as far as I know). It is isn't a good picture, but should be enough for this purpose. Maybe you can get a pinout for the Tascam jack and see if it's the same.
If this doesn't help try contacting Tascam, or the developer of the app you are using with the interface.
Edit: I should have just posted this link, which is way more informative, especially if you read the notes.
http://pinouts.ru/HeadsetsHeadphones/iphone_headphone_pinout.shtml