Legal Question in Intellectual Property in New York

Re: disclosed proprietary info in a letter to a potential competitor need protection

Don't beat yourself up too much - this kind of thing happens with some regularity. The bad news is that the ideas and plans are not, in and of themselves, ordinarily protected, unless you gave the material to the other person(s) with both sides clearly understanding that you are entitled to compensation if the ideas are used.

A certified letter won't advance the ball, and, particularly if they're represented by counsel, it's unlikely that they'll sign an agreement saying they won't use the idea. I certainly wouldn't recommend that a client sign such an agreement.

Re: disclosed proprietary info in a letter to a potential competitor need protection

You should (or actually an attorney should so they at least suspect you're "lawyered up") send a certified letter, return receipt requested, acknowledging that they have destroyed your work. However, as human minds can be "forgetful" or absorb ideas that somehow get mixed in with other ideas, you also want to assert, diplomatically, that the information to which they have had access is your intellectual property and that any use of such property without your consent may subject them to legal action (you should be prepared to do so if you write it). If you have not already done so, you should attempt to register for copyright any original material you have furnished to them, including any drawings, designs or other pictorial works. Same goes for audio and audio/visual works. If you've provided any potential trademarks or service marks that are not currently registered, you should also apply for same at the USPTO on an intent to file basis. Finally, if anything you've presented can be the subject of a patent including a "business method patent" you should seek to register as well. Be prepared to spend some money as the foregoing should be done via an IP attorney. If your property is important enough to protect, start building the weaponry to keep the barbarians outside the gate.