Hello @jennifer
Thank you for your reaching. Unfortunately, this information cannot be cleared using PowerShell or Graph. You can create an Azure AD Support ticket requesting to clear all On-Premises Attributes from previously synchronized users on your tenant. Support Team should be able to contact product group with your tenant information and help you achieve this.
I hope this answer helps to resolve your issue. Please "Accept the answer" if the information helped you. This will help us and others in the community as well