Stopping products from being indexed in seach engines

How can we stop an individual product from being indexed by all the major search engines? We've tried hiding the product but it still gets indexed… My question is if we select that you have to be a registered visitor to view the product does that stop search engines from also seeing that product and indexing it?

I know, I know, It's kind of backwards thinking and against what most of us are wanting but we have a few products we don't want out there publically…

I too would like to know the best way to do this. In our scenario, we have recently changed what we sell online, so instead of 600 y products, we now carry 50 z products.

How do we drop the extras we don't need without hurting our ranking or getting 404's?

Thanks for any advice.