The safest route is to not engage at all. What do you think is the worst thing that could realistically happen if you didn’t respond? People don’t respond to other people all the time. That’s just life. Everyone is busy with their own lives. If you don’t respond and you see this person later and they call you out for not responding, it’s easy to say, “Oh sorry I totally forgot you messaged me. I was busy with X and got distracted by Y. How are you?” And then just have a brief grey rock-y small talk interaction where you share nothing about yourself, say “glad to hear you’re well” and give an excuse of having to be somewhere else/do something else to extricate yourself from the interaction quickly.

Another option is to act like you’re very preoccupied in your life with something either dreadfully boring or unpleasant such that they don’t want to talk with you. A tooth infection. Chronic joint pain. Your dog has an incontinence problem and you wouldn’t believe all of the things you own that Fido has shit all over. So now you’re constantly replacing furniture and blankets and carpets because you can’t remove the poo stains and it’s taking over your life. You keep taking Fido to various vets and they can’t figure out why Fido has such loose bowels and putrid shits. You’ve tried changing his food but that just resulted in flatulence problems.

People like this want validation so if you make yourself appear to be an unlikely source of it, they’ll look for an easier target.