@Mikel Kaubfa You can’t prevent them from leaving and breaking the lease. (That’s why longer leases like the 2-year one you signed with them are usually not advised by experienced landlords. They tend to benefit the tenant more than the landlord.)
You do not have to let them find a new tenant for you though. You’re the landlord and it’s your property. That’s your job. Not the tenant’s. They have no incentive to find a good tenant for you (they likely don’t even know what a good tenant is).
Once your current tenant moves out, get the property rent-ready and start marketing for a new tenant as quickly as possible.
Your current tenant is legally responsible for the rent until you find a new tenant and that tenant begins paying rent. However, you have an obligation to make a reasonable effort to find that next tenant and mitigate your losses.
You don’t have to accept the first person that applies. You can apply your same/normal screening criteria and wait until you find an applicant that meets them. Once you do, and that next person begins paying rent, your previous tenant is off the hook (meaning you can’t collect double rent from both of them).
Lastly, don’t forget to send your current tenant an itemized statement for any deductions you make from their security deposit within the required 21 days (or you’ll lose your right to make any deductions). You can read more about that here in case you have any questions:
California Landlord-Tenant Handbook
Hope that helps. Good luck.