How does Radix prevent spam transactions?

The cost of sending transactions across the Radix network is currently set at $0.01, while the cost of validating a transaction is almost negligible in terms of computer resources. 

This makes the attack highly asymmetric in favour of Nodes, meaning that an attacker would have to spend a very high amount in fees to create any kind of stress on the network. If an attacker spams the network, nodes are immediately incentivized to validate them and collect fees for work done, in extreme cases, attracting more nodes to the shard being attacked due to the strong, positive economic incentives.