If you work DX you are likely to get QSL cards, even if there is no explicit request to exchange them from the other stations. They certainly mean no disrespect and view it as a courtesy (indeed, I have always viewed a request for my card from a DX station as an honor). Nonetheless, you are under no obligation to respond.
In addition, many organizations have QSL bureaus -- FISTS, 3905 Century Club are examples, among many others. If the note was from a representative of one of these organizations (assuming you are a member), then there is an implicit reciprocity at work that, many believe, should be honored.
If neither of these is the case then I agree with the previous response -- report the incident to the US Postal Inspector.