HI,

wire up the client-side ExpandedStatechanging event and cancel it. 

 function WebPanel1_ExpandedStateChanging(oWebPanel, oEvent){
 //Add code to handle your event here.
 oEvent.cancel = true;
}

As far as the image is concerned you can replace this image with another -  transparent image.

 Here is a help link to the properties

off of the Header object - there is an ExpansionIndicator property

http://help.infragistics.com/Help/NetAdvantage/ASPNET/2009.1/CLR3.5/html/Infragistics35.WebUI.Misc.v9.1~Infragistics.WebUI.Misc.PanelHeader_members.html

there is a CollapsedImageUlr and ExpandedImageUrl

http://help.infragistics.com/Help/NetAdvantage/ASPNET/2009.1/CLR3.5/html/Infragistics35.WebUI.Misc.v9.1~Infragistics.WebUI.Misc.TwoStateImageButton_members.html

 code to change the image would be similiar to the following snippet:

WebPanel1.Header.ExpansionIndicator.CollapsedImageUrl = "colapse.gif";
        WebPanel1.Header.ExpansionIndicator.ExpandedImageUrl = "transparent.gif"