File private complaint before magistrate under section 156(3) of cr pc to direct police  to investigate and submit report 

in alternative file complaint before senior tribunal and seek injunction restraining neighbour from disturbing their possession of house 

See your parents can file complaint before the District Magistrate and Superintendent of Police to take action on the said FIR and on neighbor. 

You may lodge complain before Sub Divisional Magistrate of that area under provision of Senior citizens Protection Act. You are instructed that I will not contact you but if you are interested you may call me 

Besides police complaint they can file a complaint before the senior citizens welfare tribunal of your district and also a suit for permanent  injunction against the intruders restraining them from interfering in the possession and enjoyment of the property by your parents for whatever reasons.

It is not clear what sort of harassment your parents are meted out it.

If the nature of the harassment is akin to breach of law then you or your parents can lodge complaint with Police.

if their civil rights are violated then they can file civil suit.

If Police is not acting upon your complaint then file writ petition in high court against police inaction.

Apply for the action taken report through RTI about the police complaint lodged, if they fail to respond you can escalate the same to higher officers i.e., ACP, DCP & CP of the concerned area.

Further, if you don't get any relief inspite of above, your parents can lodge complaint before the Senior Citizens Tribunal seeking Injunction against the said neighbours.

1. Parents can file a Police FIR, requesting investigation and charge-sheet, for offences like Trespassing, Cheating, Intimidation, Fraud, Breach of Trust, Mischief etc.... against the person, supported with all relevant supporting Documents, Evidences & Witnesses.

2. IF the Police does not take action, THEN Parents can file private criminal case u/s 156(3) Cr.P.C., in the local Magistrates court, seeking directions to the Police to investigate and file charge sheet. The said Person may come down for settlement.

3. Another option is file Writ Petition in High court, seeking directions to Police for Safety & Security of your senior citizen parents, from the said neighbors.

Ask them to file complain before collector under senior citizen act and pray for protection order than collector will direct nearby police station to give protection.

 if the police does not do anything then file a case u/s 133 crpc whereby the magistrate would direct the police to stop the nuisance.

For any complaint as a normal public they may lodge their complaint with the police station concerned. The age is not a consideration in this regards.

you can seek immediate police help and recording of FIR from house as senior citizens are privileged for the same.

1. If police is not taking any action then send a letter to SP of your district and go to meet him personally for complaining about police officials For not taking any action of complaints lodge against neighbors 

2. Your parents can also file suit for permanent injunction against neighbors for restraining them to enter their property.

Dear Client,

If police is not acting upon your FIR, then you complaint to the Commissioner of Police for the same mentioning your prior Complaint.