If this is so, surely an unscrupulous user can just delete whatever these files are and reset the trial each time it expires?
Correct. That's true for all unverified trials. And since most apps in the world use unverified trials, this is true for them as well.
If this is so, do I need to put in extra code around a "TurboActivate.IsActivated" call into the app that limits functionality "just in case"?
You'll need to use verified trials if you don't want to use unverified trials.
In about 2 months we'll have an improved verified trials (making it easy to use as the unverified trials in TurboActivate).
What is your recommended mechanism to make unverified trials as secure as possible?
You can't make them secure -- that's the nature of unverified trials (whether you use ours, build it yourself, or use our competitors products). That's why you should use verified trials.