Measure the length and width of the hole you suspect the rats are using. Measure the width of any pipes running through the hole as well.
Cut a piece of sheet metal that is large enough to cover the hole using a grinding wheel. Drill a hole large enough to fit around any pipes in the way as well.
Disconnect the pipes that reach into the hole. Slide the piece of metal onto the pipe. Replace the pipe and slide the piece of metal flush against the hole.
Install screws through the metal and into the wall to hold the piece of metal in place. This creates a strong barrier against the rats. Caulk around the pipe to prevent the smaller pests from squeezing through your defenses.