And you can already remove something shared with you by going in Drive to "Shared With Me" and clicking "Remove" in the context menu for the file/folder.
Otherwise, sharing something with a Google account is no different from sending an e-mail, e-mailing a Calendar invite, etc. After all, you can't turn off receiving e-mails or calendar invites.
But, like Gmail, what Drive should have is the ability to block specific sharers.
In the meantime, you can use the "Report abuse/copyright" menu item on any shared item, although I don't know if that helps. :(