Yes, you can make a counterclaim to a complaint against your business, but you need experienced legal advice to do so. If you have a general liability policy for your business, take the lawsuit papers to your agent immediately and get your insurance company involved. If the claim is covered by your policy, it will provide you an attorney and indemnify as well.
Be careful of the time limitations to respond to the complaint. Don't delay.
You would counter-sue for "defamation;"
"business defamation;" maybe other
counts (need more info). You NEED to
discuss your particular facts with an
attorney. It'll cost you a few hundred
dollars to have attorney review your
paperwork. That's if you do this "in
pro per" or w/o an attorney. You can
always just hire an attorney to counter-
sue this "disgruntled former customer."
THIS ANSWER IS PURELY FOR ACADEMIC DISCUSSION ONLY AND DOES NOT CONSTITUTE ANY TYPE OF LEGAL ADVICE OR LEGAL REPRESENTATION.