Hi,
Those actions are currently defined in the xhtml template. To override it and remove the Add gagdet action, you need to override the user_opensocial_dashboard.xhtml template in nuxeo.war/incl/dashboard/ in your own bundle and remove the following line:
<a id="addGadgetButton" href="#{baseURL}site/gadgets?language=#{localeSelector.language}">#{messages['label.opensocial.container.add.gadget']}</a>
