When I was first playing with Sylius I noticed that there is no easy way to disable a product - I'm sure you could change the stock settings or something. I prefer to have a simple on / off switch so I set about creating one by extending the Sylius Product Model, here's you can do it too:
First, create a bundle within the src folder of the sylius installation (since Sylius is a Symfony2 application it follows the Symfony2 folder structure). This is my first attempt at using Sylius / Symfony2 so I'm not sure about best practices for bundles, but having a bundle for your project that contains all of the extended classes seems sensible. Since I'm working on a super secret site, my bundle will be called 'SuperSecret'. Symfony2 uses psr0, so my folder structure ends up as src/PeterFritchley/Bundle/SuperSecretBundle.
Now that we have a bundle, we need to create a class t