I like feeding false data..
.. and usually so it does the maximum damage when abused.
For instance, when they insist you need to give them a phone number or a physical address (which, btw, is usually in direct violation of most EU privacy laws as it's deemed collecting in excess of what is required), I always pick one that would actively track down who is spamming them like, say, the ICO.
In other words, if someone starts using that data they dig a nice hole for themselves.
That said, usually I bail from sites that want that much. I reserve my energy for the local phone company which has by now racked up 3 straight privacy law violations and now it's time I ask the local equivalent of the ICO if they have actually reported them, knowing full well that that will trigger an investigation*.
* No, trust me on this, they seriously deserve all I can throw at them, and then some. By the time I'm through with them, their shareholders will ask their manglement WTF they were thinking.