Ah, OK. Thx for the additional information.
Well, there is something that can be done, but from what you have tried already, you are going to need the assistance of a lawyer.
The document that needs to be filed is a Motion to Quash. You could probably file this paperwork yourself, and you could find an example copy from the court's clerk. All courts utilize model forms. You could simply type one up filling in your specific information. However, I think a lawyer will be required so that you can not only get the current subpoena quashed but also to obtain future protection in the form of an order signed by the judge. These have many names and titles; e.g. Order of Protection, Order of Excusal, etc. To obtain a permanent excusal, you will need the help of a lawyer and obtain a a short hearing, 15-20 minutes.
Your goal would be to demonstate to the court that you truly have no relevant information in the proceeding. Obviously past letters and other evidence of attempts to be excused will become very important sucvh as letters to the person's attorney that have been ignored. Once accomplished, you should not have any problem obtaining the order.
You could also sue this person for civil harrassment, and it might be possible to sue for malicious prosecution, (the latter is rare in most states but something worth looking into). However, you decide, please try to find a lawyer through reputation and referral from people that you know. I do recommend that you at least visit with a good local attorney that does trial work. There is no substitute for that.
Please press accept and leave some feedback when you get a chance.