If your application or access to the database doesn't use dbo or sa authority you can just use:

ALTER DATABASE db_name SET RESTRICTED_USER With ROLLBACK IMMEDIATE

This will drop all user connections and rollback any non-committed transactions. When RESTRICTED_USER is specified, only members of the db_owner, dbcreator, or sysadmin roles can use the database.

We use this to restrict access to a system so we can backup Production databases prior to a deployment.:-)