If you receive the error "At least one field could not be deleted, because there is submitted form data for the field(s). All submitted forms that contain data for these field(s) must be deleted before the field(s) are allowed to be deleted." when trying to remove a form field from one of your custom forms, you will first need to delete all submitted form data for that custom form.
In order to do this, you will need to first click on the [Forms] tab at the top of the liveSite Control Panel. Next, click [list] next to the custom form that you are trying to remove the custom field from.
Then, you will need to make sure that the date range at the top of this screen goes back all the way to the beginning. After that, you will need to select all form submissions and then click [Delete Selected].
After you have done this, you should be able to go back to editing the custom form and remove the field you were trying to remove.
Note: You may want to export your submitted form data before doing this. You can do this by clicking on the [Export Data] button at the right side of the screen.