I looked at my credit report recently and saw a couple of negative items on my report. I sent the collection agencies a request for validation the debt. They responded back that it was past the 30 days that I was allowed to respond and that they will keep contacting me about the debt and keep it on my credit reports until it is paid. My question is can I still send them the validation letters and are they legally bound to reply accordingly according to the FDCA? Or am I stuck?

The company is correct.

You had 30 days from the date they sent out notification of the debt to request validation.

They are not required to produce such a document now unless they sue you, then you can require it as proof when you go to court.