I'm not clear if the goal is to prevent further trespass and damage to your crop or to extract damages of some sort. Reading the Michgan trespass laws, it would appear clear that these folks have engaed in at least "Misdemeanor Trespass" which means the maximum penalty would be by my reading (IANAL) be "punishable by imprisonment for up to 30 days, a fine up to $250 or both". I believe that orchards are specifically mentioned in the law.
IN NY State: "Even on vacant land, a written notice delivered in person (or by certified mail with a signed receipt, etc.) to any person, in the name of the landowner or authorized party, containing a description of the premises and a warning of restrictions which apply has the same effect, for that person, as if the land were posted with those restrictions."
I would suggest getting an attorney to draft a stern, certified letter to these people and any other neighbors who you believe to be trespassing on your land and stealing bluberries. This may not be part of MI's law, but will put them on notice and may help in future litigiation. The fee to a lawyer will be inexpensive to draft such a letter and then you have someone to assist you in sueing these people if it comes to that.
Keep bugging the prosecutor and eventually he may decide that doing his job is less bothersome than reading your letters and emails.