It must be a datasheet - it's the only view I'm aware of that exhibits this behavior. You can delete it if it's not being referenced by some action that happens with the form, such as running a query. If you need to keep it, just drag the right border of that column all the way to the left border. You could format the column (not control) width in the subform current event to ensure no one sees it, but unless it's top secret stuff and needs to be there, it's not worth the effort.
The more we hear silence, the more we begin to think about our value in this universe.
Paraphrase of Professor Brian Cox.