One additional thing to note is that the diligence process can be an intelligence-gathering bonanza for a larger acquiror. The information they glean can be either harmless to you (data points on employee shares/salaries allows them to build knowledge of early-stage compensation, which is very useful when poaching engineers from other companies) or harmful to you (they see your private information and act on it in a way that damages your business).
For what it's worth, I would advise people approached by Corp Dev types to not only say no as PG suggested, but do so in 10 words or less. The email response should be "No, not interested. Sorry" -- no greeting, no pleasantries, no signature. You won't hurt their feelings and a longer email is just an invitation to them to start a conversation that is going to suck up your time and energy.
If you are interested I would demand a break-up fee as others here have noted, as well as an aggressively worded NDA and a non-hire agreement that stops them from poaching your employees.
Finally, I would also consider the effects that knowledge of an explored-but-abandoned acquisition would have on your employees' motivation if they found out. Generally not good for culture and/or long-term goal orientation.