Can you please mark your previous post as private ?
I have logged in with my username and published that post again. I could NOT see the custom field.
Then, I have logged in with the username that I have gave you and indeed I could see it.
So, me as a SuperAdmin or Admin I should be able to delete the custom fields added by all the users.
This is the point of the plugin. Users adds guest post and I moderate it. Otherwise I shoud ask every user to delete a field.