Poison in bait boxes in the areas you suspect them to be. They may not touch them for a day or two as they are suspicious buggers. Put the boxes in place and wait for about 3 days before inspecting them. If they have taken the bait, re-bait the boxes. Don't check them every day, just every 2-3 days and don't move them around, leave them in the same place. Rats hate it when things change but they won't necessarily move out, they will change their routes.
When you find the bait is no longer being touched, leave it for another week or so. The adults may be dead but there may be some youngsters who haven't found them yet. Good luck with getting rid of them. Filthy vermin.
As a last resort you can call in a pesticide company who will put non-poison bait down first and then come back to see if it has been touched. If it has, they will then put down the poison bait.
