@Moritz_ 

While router firewall works only for incoming connections you can try using Parental control or Access control (check the router's user guide for details) for`restricting Internet access or website access for a particular device.

Here's a guide for configuring Parental Controls - at the bottom of the guide there's an option to pause the internet access immediately for a particular device.

If that doesn't suit your needs the other thing you could try is to configure static IP address settings on camera and leave its gateway IP address empty (if possible), then test if the camera is accessing the cloud.