That might be difficult if the drive is NTFS. Without third party software, Macs can only read from NTFS, not write to it.
I know very little about iTunes, except that it killed off all meaningful competition for music playing software on the Mac platform and left us with the steaming pile of excrement that is iTunes. How this ever made it past Steve Jobs' infamous uncompromising standards is a real head scratcher.
So, if I had to guess, I'd say your best option would be to make a new user account on your computer, then set up iTunes with the person's iTunes username/password, import the data from the old drive, then you can deauthorize it. I'm not sure if there's some central database over in Cupertino that stores authorization info or if it's kept locally on the machine, but if it's local, then you'll at least have time to work out some method for writing to an NTFS partition. If it's some mystery server in an Apple server farm, then you should be able to delete the content after deauthorizing. Then delete the account too, though sometimes it's useful to have a second account around in case something ever happens to yours. More likely on Windows than OS X, but what does it hurt to have it and never need it? It takes up maybe a couple MB of space.
I posted this question in the itunes forum a while back and received no replies so i am seeing if someone here can help. I have a drive from a defunct pc that has a family member's itunes library on it that I wish to de-authorize. Since I no longer have a pc in my household can this be done using a Mac?