You blocked a device but it still has access to the Internet.
The device you are blocking may not be the one you wanted to block. For example, if the device was named incorrectly or you have multiple of the same device e.g. an iPhone then you may be blocking the wrong device.
Check the IP address of that device with the IP shown on the device itself - usually found in the devices network settings.
If you have not enabled password protection on the NETDUMA interface it is also possible that another device has accessed the interface and unblocked it.