Hi,
In the Prices area of these B2B products, when you edit/create a price, you have an "access level" setting which allows you to restrict the price to only some user groups.
So you'll want to restrict the price of these products to the bB2B user group where your B2B users will be in.
Then, make sure that the "Display add to cart button for free products" setting is turned off in the HikaShop configuration so that other users, who won't see any price on these products won't be able to purchase them for free.
Finally, change the text "Free" that will appear on the page for them to something else like "price available only for B2B users" with a translation override:
www.hikashop.com/download/languages.html#modify