I would first suggest writing a letter to your tenants with a reasonable timeframe for correcting this issue, and keeping a copy for yourself.
The next step would be to serve an N5 Notice for Disturbing Others, Damage or Overcrowding. This is a 20-day notice to terminate the tenancy, but the tenant can void the notice by correcting all of the issues in the details section of the form within 7 days. If the mess is removed (and damage to property fixed, if that is required) then the matter is resolved. If not, then you may apply for an eviction hearing on an L2 application to the Landlord and Tenant Board on the 8th day.
The forms and instructions are available here https://tribunalsontario.ca/ltb/forms/ on the Board’s website.
If the matter does go to a hearing, you will need witnesses to help describe the problem, because you cannot rely on the photographs alone. Witnesses could include neighbours as well as property standards employees.
As an example, here is a relevant part of the Toronto City Municipal Code Property Standards Bylaw which can be found here:
http://www.toronto.ca/legdocs/municode/1184_629.pdf
Ҥ 629-10. Maintenance of yards and property.
- Nothing in this section shall be deemed to prevent the lawful storage and keeping of material in or on any non-residential property, if a lawful use requiring that material is conducted on the property and the materials are stored in a neat and orderly fashion.
- All yards and any other part of a property shall be kept clean and free from accumulations of junk, rubbish, brush, refuse, litter, garbage and other debris, and any conditions that are health, fire or other hazards…”