Be direct and don't over explain yourself.
Dear Persons Name
Thank you for applying for a modeling assignment with our company.
Your application has now been processed by Company Name.
It is important for us to follow strict policies and guidelines when hiring models for our promotions. Unfortunately, at this stage, we are unable to approve your application.
If you require further assistance or need to discuss the outcome of your application please feel free to email us: [email protected]. You should get a reply within 4 calendar weeks.
Sincerely,
Company Name
If the 4 weeks doesn't deter them, just reply with an email that says "There is an abundance of models that matched the criteria you supplied, so the system automatically denied your application. Thank you again for your application"