I'm not a lawyer, but I would have thought that trademarks are balanced by common sense and context. So, if you have a shoe company called London, and I call myself London, you have no claim against me unless I'm also selling shoes (because it seems like I'm representing myself as you), because it's a reasonably common word that has meaning outside of your shoe-selling context.
In your case 'Lindblad' appears to be a reasonably common surname. It just so happens that one person with that name set up an expedition company - I'm guessing they're the ones that wrote to you. In this case I think the company has absolutely no claim over the name whatsoever.
So, here's what you do:
Post the letter on your site's blog. Write an article about how you don't comply with baseless legal threats, and instead you prefer to shame companies that attempt to terrorize individuals into giving up their legitimate use of generic terms (in this case, a surname). Submit the article back here and to other places that are interested in legal overreach (reddit/boingboing.net come to mind). Send a link to your article as a response to the lawyers who wrote to you (or write a separate response but add this to your articles). Keep updating the article/blog with any responses they send you.
Your customers will have increased respect for you for not giving in needlessly, and you may even get a few new users who found out about you just from this. Plus you'll have struck a small blow against the stupid use of legal threats by companies.