Accidentally joined an HSA while on Medicare. How do I file this?

I can't believe I'm the only one who has made this mistake, but I can't find any information about it searching the web. Last year, I was enrolled in Medicare Part A and was talked into getting an HSA by a co-worker. I'll admit that I didn't research this at all and didn't discover the mistake until I was about to reenroll this year.

So, how to I properly report this mistake to the IRS? Is there some way to do this through TurboTax?

I've been avoiding calling the IRS mostly because I don't want to explain it over the phone, but if that's what I need to do, then I'll do it.