It's time to act independantly of Linden Lab.
If you live in the uk, find yourself a decent solicitor and grab the first half hour free. Make notes on each incident and take it with you. This will enable the solicitor to quickly read and see each issue without you having to relate the story (which takes longer).
If you have house insurance with legal cover, take the insurance policy with you. This may fund any action you have to take against this person. You may need proof of income also, ask when you make the appointment.
This is harassment, going down the defamation route may be harder than harassment. I'm also pretty certain that legal aid is rarely awarded for defamation cases, but the solicitor will advise you accordingly.
It appears that LL is failing to address your legitimate complaints. Contact LL and ask for their legal dept. Inform them that your solicitor wishes to contact them in relation to a series of incidents which have now become very serious. Get the person's name that you speak to and keep a note of what was said during the conversation. Make sure you stress to them just how much this is affecting your life, be it SL or RL.
Sometimes a phone call to the right dept can have the desired effect, but there are no guarantees.