Unlike a copyright claim that only has one potential plaintiff, EVERY disabled person in America is a potential plaintiff, as well as your state's Attorney General and the US Attorney General.
Plus, once you've been sued, you can no longer claim ignorance if someone else sues you over the same issue later.
The only proper response is to say "thank you for informing us of the problem" and if there is an actual violation, fix it or find some legal work-around (You have excess bathrooms that aren't ADA-compliant? Close them down. Is there a way to apply for a waiver? Apply. Etc. etc.). But do not pay the dane-geld or you will never get rid of the "Dane".
Oh, and yes, these guys should be disbarred for offering to settle a claim when they know good and well they cannot speak for all potential plaintiffs AND they know good and well that if a settlement is reached, it will at least temporarily defeat the purpose and intent of the law.