<a4j:outputPanel layout="overflow">
should be layout="block"
and add css class for that panel with css overflow declaration.
no use , horizontal bar is disabled. but vertical is there and it is working
<a4j:outputPanel layout="block" styleClass="scrollchk">
<rich:extendedDataTable height="150px"
sortMode="false"
value="#{mainBind.customWizard.clstrDtl.extendedDevices}"
binding="#{mainBind.customWizard.clstrDtl.dtdeviceInfoCol}"
selection="#{mainBind.customWizard.clstrDtl.deviceInfoSelction}"
var="device" id="devicesTable"
enableContextMenu="false"
selectionMode="multiple">
style class is
.scrollchk
{
width:150px;
height:150px;
overflow:scroll;
}