Since multi-selection is wanted (at least that's how I interpret this) then definitely a list box on a form. If they need a list value that is not there then one approach would be to use a command button to either update the listbox rowsource with value(s) entered into text boxes on the form, or open a form that is specific to this purpose (research list items edit form property). While a combo is easier to add list items to, it doesn't allow for multi select unless it's bound to a multi value field and as noted, you don't really want to do that.
The more we hear silence, the more we begin to think about our value in this universe.
Paraphrase of Professor Brian Cox.