Watching IOTA presentation there is a thing that is still unclear for me:

Knowing that IOTA involves PoW but a much lighter one (in order to achieve thousands of transactions per second, but also would not involve a big computational power) and also being every node that includes new transactions into a validator:

• What will avoid to a spamer/hacker post invalid transactions into the IOTA network? Let´s say, hundreds or thousand of devices starts to include new invalid TXs (transference of tokens or data) that they got confirmed by the other nodes participating in that SPAM process.

Thanks in advance!