Deleting a user in QuickBooks Online is like a walk in the park. I'll guide you through the steps.
Sign in to QuickBooks with a user profile that has permission to manage users, e.g., Primary or Master admin, Company admin with permission to manage users.
Select Settings ⚙ and choose Manage users.
Find the user you want to delete. Then select the arrow icon in the Action column.
Select Delete, and click Delete in the Confirmation window.
Here are articles that will give you more details about managing users in QuickBooks: