I once did the same thing to send a letter to immi and I got the same answer. It seems to be a standard letter to whoever making a complaint. Mind you whether they really make reference to it when approving an application is actually an unknown. But I truly hope it works.
Actually I just wrote the letter to immigration department - 灣仔告士打道7号入境事務大樓3樓外籍家庭傭工組收, indicating all the details, including the maid's name, passport number, HKID. Write down clearly what she has done wrong and request their team to review the incidence plus ask them to put your letter in file so that if there's another HK family who would like to hire the maid in the future, the immigration department has its records and let the future employer knows about the maid's history before they give out the working visa. Also write down on the letter your name, contact details. Should the have any queries, they may give you a call. After you send the letter, you should have received their reply within a month. Hope that helps. :)