Require that all prospective roommates be screened and added to the rental contract.
Prohibit residents from subletting their rental units. When a resident sublets, the new resident has no legal obligations to you under the contract.
Prohibit residents from assigning their rental contracts without your written consent and only on condition that the assignee be screened and approved by you.
Except for children and other dependents, everyone living in one of your rental units should be screened and approved and required to sign the rental contract.