Terrible situation and I am afraid the police as an organisation have a poor track record dealing with repeat ASB reports. (management issue).
How long ago was this latest offence reported Ladywriter?, the case may still be under investigation but the police have a duty to keep a victim updated on at least a monthly basis where no suspect has been dealt with, up until the point where the case has been filed undetected or closed.
Lower level criminal damage and vehicle tampering offences do not usually qualify for automatic referral to Victim Support however, as this is repeat victimisation you can request this service which may be beneficial in many ways.
Contact the force or officer dealing with the case and ask what progress has been made and whether the case has / is being referred to your local Neighbourhood policing team who work with other agencies to try and resolve these type of ongoing matters using various other methods.
If no joy then as Eddie states book an appointment to speak to the station duty Inspector and request a review of all reported matters etc.
As already stated, the stones offer no real value to the case in this type of incident.