This is a sort of awkward method, and there may well be a better way, but here's what I've done in the past
1. Set "limit to list" (properties - data tab) to "No"
2. For the "on exit" event write code along the lines of...
If Me.cboBox.Value <> "combo boxitem1" And Me.cboBox.Value <> "comboboxitem2" Then
MsgBox "friendly message here", vbOkOnly, "field not searchable"
End If
The "If" line goes through all items in the list. Works better for smaller combo boxes. If anyone else has an alternative, I'd love to hear it!
CJ