Assuming you have the following dialog
<p:dialog id="dialog" widgetVar="dlgVar" dynamic="true" >
<p:ajax event="close" update="growl" listener="#{dialogBean.handleClose}"
onstart="PF('dlgVar').content.empty()"/>
</p:dialog>
Button
<p:commandButton value="Button"
onComplete="PF('dlgVar').show()"
update="dialog">
or you can call PF('dlgVar').content.empty()
in the onHide
of the dialog, if you don't need an ajax request.
<p:dialog id="dialog" widgetVar="dlgVar"
dynamic="true"
onHide="PF('dlgVar').content.empty()">
</p:dialog>