This is possible for a single user login as you can only asign ownership of a database to a single user.
Should be ok for most small hosting situations. This is how I achieved it under SQL server 2008...
1. Right click the users logon under the users database if they exist and remove
(Note: Do not remove the user from under the main security login)
2. Run the following query
USE <customersdatabase>
ALTER AUTHORIZATION ON DATABASE::<customerdatabase> to <customerlogin>
USE MASTER
DENY VIEW ANY DATABASE TO <customerlogin>