Hi,
By disable I presume you just mean hide the Start Discussion actions?
If so, this can be done by hiding the appropriate actions in your web-client-config-custom.xml file.
Add the following to your file and you shouldn't see the actions anymore:
<action-group id="document_browse">
<show-link>false</show-link>
<action idref="discuss_node" hide="true" />
</action-group>
<action-group id="document_browse_menu">
<action idref="create_forum_node" hide="true" />
</action-group>
<action-group id="space_browse">
<show-link>false</show-link>
<action idref="discuss_node" hide="true" />
</action-group>
<action-group id="space_browse_menu">
<action idref="create_forum_node" hide="true" />
</action-group>
<action-group id="doc_details_actions">
<action idref="discuss_node" hide="true" />
<action idref="create_forum_node" hide="true" />
</action-group>
<action-group id="space_details_actions">
<action idref="discuss_node" hide="true" />
<action idref="create_forum_node" hide="true" />
</action-group>