I created a WebTemplate in Visual Studio and deployed it as a farm solution (I couldn't create a sandbox solution because it needs to deploy a custom master page and other stuff to the GAC). Now it shows up both in Central Admin, on the create site collection page and within existing site collections, on the Create a site page. I want it to be available only when creating sub-sites; I don't want it to be used for a root site.
Is this possible? How?
as per Naim's answer, I tried adding SubWebOnly="TRUE" in the ONET.XML inside the
<Configuration> tag, but the template still shows up in Central Admin Create site collection page