I did a search for this phrase: “wordpress remove custom field value from the drop down menu”. A bit verbose perhaps, but it got me this link: http://sumtips.com/2011/03/add-delete-edit-custom-fields-wordpress.html
From that page, the author states that you can do one of the following:
- Delete Custom Field from all posts.
- Delete it from the database.
- Delete it using a plugin.
There’s a lot more detail in the link above, but to not steal their thunder, I’ll let you following the link itself.