If your name is going to be the only one on the contract, the landlords/property managers will notify the co-tenant that s/he will be required to be out by a certain date, as a 'new' tenant will be taking over. You can definitely tell your roommate that you will be signing the lease by yourself starting August. But you do not own the property, you cannot evict anyone. The landlords will be responsible for giving proper notice to any other current tenants/