In that example you'll see some code like:
infoControls = {
click: new OpenLayers.Control.WMSGetFeatureInfo({
url: 'http://demo.opengeo.org/geoserver/wms',
title: 'Identify features by clicking',
layers: [water],
queryVisible: true
}),
hover: new OpenLayers.Control.WMSGetFeatureInfo({
url: 'http://demo.opengeo.org/geoserver/wms',
title: 'Identify features by clicking',
layers: [water],
hover: true,
// defining a custom format options here
formatOptions: {
typeName: 'water_bodies',
featureNS: 'http://www.openplans.org/topp'
},
queryVisible: true
})
};
This sets up the getFeatureInfo request on the water layer - you need to modify your code to have just the layer names that you want to query in those spaces. That is don't include your raster layer in that array if you want to exclude it from the request.