Hi @Kevin Koschewski ,
Thanks for your post! Currently there isn't a built-in way to completely prevent users from creating a free subscription. If you have an Enterprise Agreement, you can create a ticket to have a Microsoft engineer block subscription creation from anyone with your custom email domain. This will only work at the tenant level and not on a per-user basis.
There is a plan to improve this experience but it is still in the works. If you would have a particular use case though, feel free to let me know and I will surface this with the product team. You can also leave feedback directly in the feedback portal. https://feedback.azure.com/
If the information addressed your question, please Accept the answer. This will help us as well as others in the community who may be researching similar information.