It does delete them in the sense that they no longer appear when you start typing. They are, in effect, deleted. The original poster did not ask about how to edit the list, which is an entirely different question (and not possible).
The list is built dynamically from the contents in the column. Every entry in the column is on the list. If you want one to not be on the list, delete every instance of it from the column and it will no longer appear on the list. There is not a separate list you can edit.
If you want a short (up to 250 items I think) menu of allowable entries to pop up and want to prevent anything other than those to be entered, use a pop up menu.