Though long associated with House Harkonnen, House Rabban was commonly seen by their superior as a lesser, distasteful ally of little value. This feeling was particularly strong with Baron Vladimir Harkonnen.
However, it is likely the Baron's opinion was tainted by the connection his hated half-brother Abulurd made with House Rabban. After Abulurd had been given control of Arrakis, and had proved a failure at managing the troublesome fief, he was sent in disgrace to Lankiveil. After his fall from power Abulurd petitioned the Landsraad to give him control of the planet, and allow him to take the surname of his wife, Emmi Rabban.
Though the exact fate of House Rabban remained unclear, it is likely they did not survive the Ascension of House Atreides to the Golden Lion Throne. The last defacto ruler of House Rabban, at least of any note, was Glossu "the Beast" Rabban, the son of Abulurd who disclaimed and later murdered his father.
Sources: Dune: House Atreides?