Hi again, as Birdie says, if the 'victim' in this offence is unwilling to give evidence then it is unlikely that the case would proceed if you ask for your statement to be withdrawn. If that happens then as far as you are concerned the case should be over.
In respect of the support you are receiving from your local Force, I would suggest two things, firstly contact your local Victim Support office, they deal with both victims of crime and witnesses to crime and may be able to offer you further advice. Secondly, if you feel that your complaints of harrassment/witness intimidation are not being taken seriously be the police then go to your local station and ask to speak to a Sergeant or Inspector and explain your concerns. Ask for patrols to be increased in your area. The OIC could also call round and have a 'friendly word' regarding his conduct.
Lastly, if you are seriously feeling harrassed by these people, please consider my suggestion regarding the video camera and keep a diary of events listing times and dates when you have seen these people throwing things in your garden or causing damage. This will certainly assist the police in investigating your complaint.
Hope this assists and good luck!