seale the place tight... then gas them dirty rats! or.

i've watched some exterminator shows over the years o discovery channel. there's a trap for rats called "jaws". these guys swear by peanutbutter as bait.

next set the traps along the inside foundation walls. rats have very poor site, so they hug walls while walking. you won't find a rat out in no rats land. set 10 of these aday. also around the entrance to the crawl space you found.

or afix a flashlight to a 22 rifle and go in, blast them dirty rats!