This is something you can play with. If you want to keep them tied on their planet/system exclusively, you might want to do something that prevents them from being x miles outside the atmosphere/system or something like that, or alter their hive mentality so that they never want to leave (such as changing something in their pheromones and so on), etc. You could make the xenophobia so bad that they never even want to leave their system. You could create a compound that interacts with their chemistry that means they can't leave an atmosphere of their specific chemistry, or make something unique to their planet--make it so that only their planet has the one thing they need to survive--a chemical, a bacterium/good virus/etc., the specific wavelength of their sun or planet magnetic field (though that could be reproduced I guess on a ship) etc.
Anne McCaffrey's Crystal Singer does it with a symbiot--once you're infected with it, you can't leave the planet for very long if at all.
If you do something to only keep them off Fondor, they can go other places.