Its not possible to do that. Nodes cant know whats in a chunk of data that comes by and from whom it comes. But freenet is to host website like sites called freesites. You cant really spam anyone with sites you can create as many as you want but if no one visits them they will eventually just "fall out" of the de central storage.

Technically you could probably bring it down if you simulate a massive amount of users who only access spam sites but that would probably become rather expensive because you would attack your own nodes and your own nodes would try to help the network. It would also need to persist forever else everything would simply go back to normal shortly after the attack is stooped.