You cannot answer a complaint with a letter. If you do not answer the complaint by filing a legally-recognized response, a default will be taken against you and a judgment later rendered against you where you will have no ability to defend.
If the entity being sued is a corporation without assets, and you are absolutely convinced that there is no way the plaintiff can come after you personally under a theory of alter ego or fraudulent transfer, then you can simply allow a default to entered against the company. However, before you do that, it would make sense to consult with an attorney who can review the situation and tell you if that is a safe move or not.
Good luck to you.
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i would strongly recommend that you speak with a bankruptcy attorney about this before deciding on a course of action. People often do not realize the full impact of a default judgment and the wide latitude of collection remedies available to creditors. The money you spend on good advice will be well worth the money down the road.
If you want to proceed without an attorney in a civil lawsuit you should research what is involved in answering a complaint.
Even if you choose not represent yourself I would adivse at least consulting with an attorney.
Good luck with your situation.