It is an offence under the Residential Tenancies Act for a tenant to change the locks and bar the landlord’s access to the rental unit. A landlord may write a letter to the tenant requesting that the changed lock be removed. If the tenant does not comply, a landlord may contact the Rental Housing Enforcement Unit at the Ministry of Housing at 416-585-7214. They can notify the tenant that he or she is committing an offence under the Act and can be fined. Read more about the process at this link: https://www.ontario.ca/page/solve-disagreement-your-landlord-or-tenant
If that does not get the tenant’s attention, you can serve an N5 Notice to Terminate Tenancy. When you issue the N5, you have to be as detailed as possible when describing the issues and include dates and times for when the incidents are occurring. The N5 is a 20-day notice but the tenants have the first 7 days to correct the problems (by giving you keys or changing the lock back, in this case). If the problems are not corrected in 7 days, you can file an L2 application at the Landlord and Tenant Board to go to a hearing and ask for termination of the tenancy.