You can enable and disable a self-installing event handler by setting its EventInfo.Mute or EventInfo::PutMute property.

var cloEvents = Application.EventInfos;
var oEvent = cloEvents.Item("myOnSelectionChange");

if ( classname(oEvent) == "EventInfo" )
{
	oEvent.mute = !oEvent.mute;
}

Users can also mute events from the Events tab of the Plug-in Manager.