In portal.properties, the following property lists portlets that can be embedded on any page:

portlet.add(default.resource.check.whitelist=3,56_INSTANCE_0000,5S,S2,S6,87,88/103,113,145,164,

The property is listed in liferay-hook_6_l_0.dtd as a property that can can be overridden with a hook.

The recommended way to prevent embedding the Search portlet ("3") is to create a hook plugin.