
Keep anything edible (and where rats are concerned thats not just things labelled 'food') locked away in metal containers.
Prevent their access - rats like little narrow holes and runways in dark corners.
Avoid giving them somewhere to hide, see above, keep everything as open and bright as you can, regularly move things like bales of paper bedding so no one fancies setting up home under or in them.
As sam says, terrier, or big vicious cat.... good quality snappy traps in prime locations.