I must allow only mobile products on my small site, for example rim, ipod device-touch, apple iphone, Android, etc.

How do you make sure that only individuals products get onto this site, which other customers are rerouted to some custom "no-access" page?

Can there be a good way using Ruby on Rails to achieve this?

You need to take out a request header known as "User-Agent" and find out whether it matches the products you've mentioned. Otherwise then redirect towards the appropriate page. Yes, you can do this with Ruby on Rails.