Well there are various way (that can be mixed):
1) - eliminate Mussolini, while after the war the rise of the fascist movement was more or less inevitable, it was he that created an unified movement from various disparate faction at odd with each others; even succeeding in convince the monarchy, the landowner and the industrialist in support him as a bulkward against communism, so without him another less capable figure will take the fascist leadership (probably D'Annunzio) and the movement will see his end with a wimper (maybe a bang if they tried a much less organizated 'march over Rome') and lot of internal conflict.
Benny was a soldiers in the great war and during his only serious action was severly wounded so it's very easy to make him not come back home
2) - eliminate ammunition for the fascist and communist, kill the 'mutilated victory myth'; basically the negotiation at Versailles will not see the utter humiliation of Italy, the dragging of the talk with the Kingdom of Serbs, Croats and Slovene about the divison of the spoils of war and more impotantly the idiotic meddling of Wilson as he greatly took future Jugoslavia side. Keep the italian delegation in Paris instead of her OTL retreat and greatly humiliating return, so to partecipate in the division of the Ottoman Empire and the German colonial empire. It's not needed to give Italy a great empire, what obtained OTL in the 20's and 30's from UK and France to smooth thing with Benny will be sufficient and regarding Jugoslavia the Tardieu plan (first draft) will be acceptable, what's important is the time factor, no dragging, no italian liberal goverment looking weak so quick negotiation and the nation will can move on and start rebuild. Better for the negotiation if D'Annunzio attempt to get control of Fiume is quietly stopped.
3) - As RyuDrago said, make the King (and the rest of the liberal goverment) grow a pair and declare martial law; the fascist are basically bluffing they don't have the resources and the weapons to effectively take power and more importantly except Benny, all the movement leaderships is there so they can be take out with a swift move. What scared the enstablishment was that while they can fight a revolution (fascist or communist), fighting another was deemed unwinnable and so making an agreement with Benny (who many thought that was easy to control
) was the lesser evil (and the communist the greatest).
In reality the communist were not in shape to really start any succesfull revolution and were deeply divided among themselfs.