OK.... it seems that by your own admission the issue is people using your wifi?

It is not common AFAIK for malware to download a huge amount of data - if anything they are more likely to transmit data. Some big culprits in bulk downloads are iTunes and other media content applications. Can you provide more information about the data usage; what is your ISP limit? How much are you downloading?

It may be time to change the security on your wifi also rather than obliterate your hard drive.

Im intrigued; lets sort this out for you.