"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
ALTER AUTHORIZATION ON DATABASE::<customerdatabase> to <customerlogin>