It sounds like validation to me if they sent you a copy of the statement from the apartment complex showing the charges. Often, they will also send a copy of the lease. If they did not send a copy of the lease and you think these are not your charges, you may want to send them another letter telling them that this is not your apartment debt and asking for a copy of the lease. If this is your debt, you are not going to get out of the credit reporting or the debt by dispute letter tactics. That said, the debt may well be uncollectible for a couple of reasons. The complex may not have properly notified you after move out, or the debt may be beyond the statute of limitations for a lawsuit, assuming that the complex did not sue at the time of the move-out.
I am licensed only in Texas. Offering information of a general nature in response to a question is not intended to be legal advice in your state.
I agree with Gary's response. In addition, you can try these options:
If you have other debts in addition to this one, you should consider a bankruptcy consultation. Many attorneys, including myself, offer a free bankruptcy consultation. Bankruptcy may allow you to eliminate most or all of your debt and allow you to keep most or all of your property.
In addition to bankruptcy, you can try submitting an online dispute to the credit reporting agencies. They have 30 days to respond. If they don't respond, the credit bureau is supposed to take it off.