Well, you can seige -- pillage all of their food tiles and fishing boats, kill or steal any workers or work boats that try to repair, and wait for the city's population to shrink after they exhaust their stored food. As the city's pop shrinks, its defense strength also shrinks. As with all seiges, it requires the attacking force be prepared to wait out the city's inhabitants and deny all relief from the outside. In Civ, that equates to a decades or centuries long seige (depending on what era you are in)....think, the Trojan War or, in more modern terms, the German seige of Leningrad in WWII (which didn't work so well for the Germans, but only because they withdrew as they started losing elsewhere in Russia).