"Determines whether to accept the key or not" in the context of the KeyDown event means disabling the action that is done from the KeyDown event, but for CTRL+V you should suppress the key press. Below is an example of how to disable CTRL+V in the fresh iGrid.NET v11 that provides the SuppressKeyPress property in the event args of the KeyDown event:
Private Sub IGrid1_TextBoxKeyDown(sender As Object, e As iGTextBoxKeyDownEventArgs) Handles IGrid1.TextBoxKeyDown
If e.KeyValue = Keys.V And e.Modifiers = Keys.Control Then
e.SuppressKeyPress = True
You can do the same for SHIFT+INS and other keys.
Disabling any char input is implemented with the help of the TextBoxFilterChar event this way:
Private Sub IGrid1_TextBoxFilterChar(sender As Object, e As iGTextBoxFilterCharEventArgs) Handles IGrid1.TextBoxFilterChar
e.Char = Chr(0)
Prohibiting insertion may require other measures - for example, processing the textbox context menu with the paste command. If you cannot implement all this using the built-in iGrid tools but know how to do that in another WinForms control, you may think about implementing a custom editor inherited from the iGCellEditorBase class.