Legal Question in Business Law in California

It is impossible to give you a clear and meaningful answer without more clarity in the question. Why might an operating agreement be necessary or useful? These are most common in limited liability companies. What kind of project? Starting and running a business? If so, was any business entity formed, or contemplated? A corporation? Or, did the parties form a partnership of some kind? Forming a partnership can happen without anything in writing, or even an oral agreement.

I just don't have enough information to even guess at the relationship of the parties or whether either of (or any?) them has incurred any legal obligation. Further, I can't tell whether there was any obligation not to quit, or what was being quit.

Please re-ask your question providing more information about the nature of the investment made or proposed to be made, the entity or group that received or was to receive the investment, and what you understand "quit the project" to mean, from a legal standpoint.

Incidentally, I would guess that "sending three separate communications" (whatever that amounts to) is not harassment and that the investor is probably trying to enforce some legal right after withdrawing from some nebulous "project", whatever that may be.

From a business stanpoint do you really want to send a cease and desist letter to an investor? if you battle him, he may demand all his money back and how does that affect the business if the capital has to be paid to him?