GridView with AutoCompleteEditor: Limiting token to a single value

I've done some searching but was unable to find anything to help my specific case. I am using an AutoCompleteEditor class to enable drop down textboxes on my grid view, but I want to limit the token to 1 min and max. How can I do this? You may ask why I am using this instead of a ComboBoxColumn. The answer is: I have been instructed to not show any drop down arrows. 

Here is the class that I've taken from forums:

  class AutoCompleteEditor : RadTextBoxControlEditor
    {
        protected override Telerik.WinControls.RadElement CreateEditorElement()
        {
            return new RadAutoCompleteBoxElement();
        }

        public override void OnKeyDown(System.Windows.Forms.KeyEventArgs e)
        {
            RadAutoCompleteBoxElement element = this.EditorElement as RadAutoCompleteBoxElement;

            if (element.IsAutoCompleteDropDownOpen)
            {
                return;
            }

            base.OnKeyDown(e);
        }
    }

I am trying to validate data entered into the cell, but it looks like end users are able to select multiple values. I want to completely disable this ability and allow only a single value to be selected.

It would be much simpler to use a regular ComboBoxColumn, but I am unable to convince decision makers other wise.