From Wikipedia:
The Invisible Internet Project (I2P) is a computer network layer that allows applications to send messages to each other pseudonymously and securely.
How can I detect/ block i2p traffic? Is it possible?
I2P is a broad network overlay and different applications can use different parts of it as they see fit. It's an "idea", not a "protocol".
As such, it will be difficult to make a broad statement about blocking "I2P". It would be possible to block various parts of the network that use identifiable traffic.