In this particular case, setting the IsAIEnabled property to false does not hide the PromptInput element in the following themes:

- Aqua
- Desert
- HighContrastBlack
- Office2007Black
- Office2010Silver
- Windows7

As a workaround, the PromptInput element can be removed by setting its visibility property to Collapsed:

gridView1.GridViewElement.PromptInputElement.Visibility = ElementVisibility.Collapsed;