Programmatically determine AWS account Id of a particular IAM user

I have a web tool which allows IAM users to do magic with their AWS account. Considering I have a free plan, any AWS user could create new IAM users once the trial has ended, so they would eventually never pay for my service. Is there any way to know the Aws Account Id of a particular IAM user? That way I can store that ID on db and block any other new IAM user coming from that particular AWS account.