Easy: block them.
It really is that simple. Twitter is too precious a resource to be ruined by the antisocial behaviour of a persistent minority.
And it is a minority – most of the people you meet and engage with on Twitter will be courteous and civil. But a small percentage of every community – and this is certainly true across the internet – will be rude and disruptive, going out of their way to poison the well.
Thankfully, peace of mind is just one click away.