What you can do is create a category (container of forums) for each sport, and you can have multiple related forums for a sport under that category. Then, on your site, you will link to categories only. This way, users won't see other forums coming from your site unless they go back to the home page.
Or, you can create the forums as "Private" and handle user registration on your site. This is how it works:
We have a site integration package (SIP) that allows you to inject users in the database as well as assigning group membership. Once you have created the private forums, you create a user group for each forum (or category of forums), so that the forum (or category of forums) will only be visible to the group.
Then, when users register on your site, you will use cookie or something to note down which sport section they come from and use the SIP to register + assign group membership.