In Magento 2 Marketplace module, the admin can disapprove the seller whenever required. For this, the admin will navigate to Marketplace Management > Manage seller as per the below image.
After clicking the Manage seller menu option, the admin will get navigated to the Manage seller page.
Here, the admin can see a complete list of the sellers. Select the seller from the list whom the admin wants to disapprove as a seller.
After this, the admin gets navigated to the Customer Information page as per the below image.
Here the admin will click on the Do You Want to Remove this Seller? menu option. And then to disapprove the seller, the admin first checks the checkbox and finally, saves the configuration.
After saving the configuration, the user will no longer be the seller. The user will become the customer now.