Anonymous letters etc

Hi everyone,
This forum seems to be the only place where legal stuffs in HK are discussed so I really hope someone will be able to help.
It's a long story so I will try to make it short.
We have been receiving anonymous letters for a few weeks now. We know who it is but unfortunately we do not have any tangible evidence of it.
We are wondering if it is possible to fill a claim against unknown person first (it is possible in other countries)? Would the police help in finding evidence as we know who it is but cannot sue him before having evidence? (we also have ideas on how to find but as an individual it is very hard, and expensive...)
The containt of the letter is defamatory (individually not against company) and also has sexual contents (pictures). The letters have also been sent to other people in our network (some being families, imagine the letter opened by kids...)
Is there only one way to deal with that: stay quiet and wait for the guy to be tired?
What would you do?
I must add that unfortunately we cannot afford to pay a long procedure whereas the sender can...
Many thanks for reading and helping.

If there is no criminal component, the police will not help.
If there is a letter of 'defamatory' but not going to be public, it is not even of civil action amendable.
And if that 'defamatory' thing is based on a fact, or with reasonable explanation that the sender think that is real, it is not defamatory.
For sexual thing, the line is blur.
I think you need to find that person and collect solid evidence he is sending the letter, otherwise you can only wait for him to get tired or escalate. And there were a few private investigator around that can help.

Thanks Dordorothy.
Actually I have news. The police filed the case as a criminal intimidation one.
Is there anybody here with experience at district court for criminal cases? I'd like to discuss about it with you. You can send me a private message.
+ any recommandation on solicitor that would not break the bank... :-/
Many thanks!