For Admin portal version 1.25 or higher
Quick navigation: Administration > User management > Select User > Delete user
To delete a user, expand the Administration menu on the lower left side and click on User Management. Proceed by clicking on the user you want to delete, so that the user information shows up, then click on the trash can icon labeled Delete user.
You will be presented with a pop-up question prompting you to confirm that this is what you really want to do. Confirm with Yes and the user will be removed.