As a business, the onus is on you to provide a service that incentivizes consumers to move up from the free tier. If your free trials are being abused, that's an issue with the way your business is structured, not how people register.
You are well within your rights as a business to decline potential customers over something like this, but you need them more than they need you.
You are well within your rights as a business to decline potential customers over something like this, but you need them more than they need you.