Legal Question in Criminal Law in Texas

Unless the communication was obscene or threatening, this doesn't qualify as harassment under Texas law. As to the issue of impersonating a public official, the State can convict someone of impersonating a public official if the person (1) impersonates a public servant with intent to induce another to submit to his pretended official authority or to rely on his pretended official acts; or (2) knowingly purports to exercise any function of a public servant or of a public office, including that of a judge and court, and the position or office through which he purports to exercise a function of a public servant or public office has no lawful existence under the constitution or laws of this state or of the United States. I'm not sure that sending you a letter with a false return address without anything more violates this statute.

I would recommend speaking to some local attorneys regarding getting a protective order against this person. This order can put them on notice that no communication is permitted to you, even if it's not threatening or obscene.