I got hit by a patent troll a few years back. I used the same technique that I'd used when MSFT once approached us and said they thought we might be infringing on their IP and could we prove that we're not. And again when another large company said we needed to change our logo because it looked like we had dotted a capital 'i', and they owned that. Just ignore them. We got one additional letter from the patent troll, and that was the last ever heard. As someone else has said, these people are looking for deep pools of money.
In the first instance, ignore. If they demand that you must do something by some certain date, ignore. When they send the follow-up, ignore. If they come back a third time, then send a really badly written letter produced on a manual typewriter or written in crayon with a hand-written envelope telling them what they're claiming isn't applicable, but provide no details. Make yourself look small, impoverished and hard work.