[SalesForce] Prevent users from adding more than one Products to an Opportunity

What you can do is create a rollup summary on the Opportunity that counts the number of products that are added to the Opportunity. Then, with validation rules, you could prevent the progression of the Opportunity Stage to anything other than the default stage when that count field equals zero.

In your validation rule you can also check the $Profile and $User globals to restrict only certain profiles or User attributes.