What "business practice"? Why do you believe the former client's conduct in dealing with you constitutes a "business practice"?
Not sure what that means. Are you saying you still have the ability to access this former client's business systems despite the termination of your relationship? What was it you were saying earlier about "unethical business practice"?
Of course you can. Anyone can get sued by anyone for anything. Actively and intentionally interfering or attempting to interfere with your former client's relationships with its clients is a great way to get a very large (and possibly non-dischargeable in bankruptcy) judgment against you.

