The first way to tell Sift an IP address or device is bad is by sending a Bad Label the users who are using that IP address or device. Sift will learn this IP address or device is associated with bad users, and other users who share that IP or device will have their score penalized. The more Bad users who are associated with a device/IP, the higher the Sift Score will be for other users on that device/IP.
Finally, we offer a Device Fingerprinting product which offers an easy way for you to block fraudulent users on a device basis from -- among other things -- using stolen credit cards and creating duplicate/fake accounts. You can view information on Device ID integration here.