1. Why do you want to email? Straightaway do a police completely for criminal trespass against him 

In that case, it would be better to serve a legal notice upon him through your counsel (advocate) rather than you sending email yourself. Your email won't have any effect at all. The counsel's legal notice may act as a deterrent. 

File police complaint of criminal intimidation under section 506 of IPC against contractor 

Email won’t suffice 

contractor would continue to harass your family 

This type of threatening activities adopted by the interior designer is illegal and a n nuisance.

You can take criminal action against him through police by lodging a criminal complaint mentioning the details and accusing him to trespass the property and creating nuisance in your property, posing threats to workers and the owner of the house etc. 

You should not get scared by the threats posed by him, you can immediately contact your lawyer and get a complaint drafted neatly mentioning all the incidents that took place including the recent threats and unlawful activities committed by the designer and lodge it before the concerned police station in the local.

If the police is not cooperating or reluctant in taking any action to thwart his attempts in future, you can approach commissioner of police office with a similar complaint seeking his intervention and direction to the local police for an effective action against the troublesome designer.

You know the seriousness of the problem, hence you are the better judge of the situation.

If you do not want to take any legal action through police then he will take advantage over the loneliness of your family at home and may indulge in more vigorous illegal activities.

You may ask them to take the assistance of a local lawyer and proceed as suggested iif you want them to live freely without such problems in future.

A criminal complaint must be filed against him with the police. If they refuse file a criminal complaint against him before the magistrate.

Send him a legal notice and askhim to cease and desist or face the consequences.

Lodge complaint against the previous contractor and register FIR for committing house trespass and criminal intimidation.

2. File a case before the consumer forum and seek damages as well.

You can file a police complaint if his harassment is criminal in nature for rest you can approach consumer court against him for deficiency of service or if he files any civil suit you can contest the same

Since he entered your flat without your permission then this is tantamount to house trespass, for which you can get the FIR lodged against him under Section 452 IPC and also 506 IPC for criminal intimidation.

Was there an agreement entered into between you and the designer?

What is the settlement amount pending from your end to him..

What are these settlement dues which he is claiming. Have you issued a closure letter to him for the termination of his services. You can send him a legal notice through an Attorney and warn him against his entry the next time he wishes to enter your premises without your permission.

Hi, it is better you can lodge police complaint against him. Writing e-mail to him will not helpful to you.

there is no  trouble in filing police complaint. police will help in resolving the issue and there is no compulsion of pursuing the complaint. if your family feels that they do not want to pursue then they can withdraw their complaint anytime. but in the given circumstances you should lodge a FIR to protect your family. or engage a local advocate and he will help you in tackling with the interior designer. 

if you want to write an email then tell him not to disturb you and if he came to your premises then you may be forced to call police. mention that you were not satisfied with his work and he cannot demand the full amount more so when the work was not completed. tell him that you will file a claim of 20 lakh damages if he harassed your family. and that if he wants to get payment then he should have worked properly.

good luck