Politics can be messy, I know. But at the end of the day, Italy could stop paying their dues, not send delegates to Nato, and never vote on any Nato resolutions. Effectively, they would not be a member and would no longer have any input into what Nato does.
I don't see how Nato could force them to send a delegate, or force them to vote, or force them to pay any dues. I think it would be detrimental to Italy to align itself with the communist bloc, however, that is their decision to make. I do not know the ramifications of such an action regarding their place in the EU........in any case, Communism does not have a stellar record regarding human rights and personal freedoms