You have to edit two macros:
1-
send_subscription_email in order to remove the reply email address, so that the subscriber won't be able to reply the message, he will receive a "no-reply" address at the
from field instead:
<override_macro name="send_subscription_email" parameters="node_attr,text_part,html_part" requires="subscription">
<n.set_local_subscription.this_subscription/>
<n.set_local_node.node_attr/>
<n.block.>
<n.new_email.>
<n.send>
<to.local_subscription.user.user_email/>
<to_name.local_subscription.user.name/>
<from_name><n.local_node.owner.name/> [via <n.root_node.subject/>]</from_name>
<subject.local_node.subject/>
<text_part.text_part/>
<html_part.html_part/>
<set_headers_for.local_node/>
<bounce_to.local_subscription.user.bounces_address/>
</n.send>
</n.new_email.>
</n.block.>
</override_macro>
2-
app_dropdown_later in order to remove the option from your app's menu:
<override_macro name="app_dropdown_later" requires="servlet">
<n.javascript_response/>
<n.get_node_from_parameter.>
<n.menu_subscription/>
<n.menu_edit_name_and_description/>
<n.menu_change_type/>
<n.menu_change_appearance/>
<n.menu_change_domain_name/>
<n.menu_mailing_list_archive_settings/>
<n.menu_change_language/>
<n.menu_change_title_and_meta_tags/>
<n.menu_extras_and_addons/>
<n.menu_use_google_analytics/>
<n.menu_create_sub_app/>
<n.menu_manage_pinned_topics/>
<n.menu_manage_sub_apps/>
<n.menu_parent_options/>
<n.menu_change_permissions/>
<n.menu_manage_users_and_groups/>
<n.menu_manage_subscribers/>
<n.menu_manage_banned_users/>
<n.menu_embedding_options/>
<n.menu_download_backup/>
<n.menu_delete/>
<n.other_menu_entries/>
<n.menu_sysadmin_options/>
</n.get_node_from_parameter.>
</override_macro>