The regulations for parking are contained in O.Reg. 561/06. They can be found on Service Ontario’s e-Laws website: http://www.e-laws.gov.on.ca/html/regs/english/elaws_regs_060516_e.htm#BK17
The first thing you should do is send the tenant a letter asking him to have this vehicle removed from the property. If that doesn’t work you should not have the vehicle towed, you should instead serve the tenant with a Form N5, he will have a seven day period after this notice is issued to still correct the situation. If he does not comply then you can apply to the Landlord and Tenant Board to make a decision on this situation. You can obtain this form from the Board’s website at http://www.sjto.gov.on.ca/ltb/.
The Residential Tenancies Act does not have specific rules about parking. A tenant may allow the use of his parking space to another person providing that it does not interfere with the reasonable enjoyment of the premises by the other tenants or the landlord.
A landlord cannot institute a fee for the parking if the tenants have had the parking facilities since the beginning of their tenancy, even if it was not specified in the rental agreement.