It's covered in the help for AddItem. Prefix the item you want to disable with "\".

PUBLIC ofrmListExamples
ofrmListExamples=NEWOBJECT("frmListExamples")
ofrmListExamples.Show
RETURN

DEFINE CLASS frmListExamples AS form

 DataSession = 2
 Top = 0
 Left = 0
 Height = 262
 Width = 325
 Caption = "List and ListItem Array Example"
 Name = "frmListExamples"


 ADD OBJECT cbolistgood AS combobox WITH ;
  ColumnCount = 3, ;
  ColumnWidths = "100,100,50", ;
  Height = 22, ;
  Left = 111, ;
  Sorted = .T., ;
  style = 2, ;
  TabIndex = 2, ;
  Top = 86, ;
  Width = 147, ;
  Name = "cboListGood"


PROCEDURE cbolistgood.Init
  WITH This
   .AddItem( 'Cleveland' )
   .AddItem( 'Buffalo' )
   .AddItem( 'Milwaukee' )
   .AddItem( '\International Falls' )
  ENDWITH
 ENDPROC

ENDDEFINE

>
>I'm filling up a combobox with 5 elements. Now i want that the 4th element can't be selected by the user. Is that possbible ?
>
>thnx