I do not know any way how they could ban anyone. But it wouldn't be necessary. verifying that a message is false and ignoring it is a really really quick operation.
If you are broadcasting too many invalid transactions/blocks to any singular node, the node will ban your IP once it reaches the ban threshold. It takes up your precious bandwidth to be receiving invalid transactions repeatedly.
And Kwothe117, you would not have to send your false guesses to the network. You could just check if your random guess is correct by yourself on your own computer. It would be much faster too.
Then if your random guess is correct you could broadcast it and it will be accepted in to the blockchain...
Random guess for transactions? Wait how?