By default, affiliate referral links are generated using 7 random symbols (e.g. https://your-site.com/?ref=nlmjdis). When this option is enabled, your affiliates are also (by default) able to customize their affiliate links on their end.

However, you can make it so that this link cannot be modified by the affiliate. 

To disable "default" links, all you have to do is go to Settings > Program > Advanced.

Here you will find the option to toggle off the default referral codes.