Close does not currently have a way to completely hide certain leads for certain salespeople. However, you can set up User Roles to limit users' ability to delete leads or edit/delete other users' activities within Close.
All leads within an organization are visible because Close is designed for a collaborate sales process. We believe this can work for most companies, especially when combined with User Roles.
You can also create Smart Views per-user so that – when using the Smart View – each salesperson only sees "their own" leads. Smart Views are helpful for workflow clarity purposes, yet it doesn't prevent access.
Also, see Lead Assignment.