It's why I have the "general rule" of: Don't reply to any text from anyone that I don't know. And if what I'm receiving "seems off" reply with a subtle "challenge" (mention a fictional pet that passed away ... the scammer will play along, your friend will probably send you a "?" to which you can ... carefully ... explain "I wanted to make sure you weren't a Nigerian Prince posing as Alice").

I know my personality would make it very tempting to reply to a message coming from the person who stole my phone, maybe taunt them a bit, but you're kind of giving into their game when you do. I mean, best you can do is burn a little of their time, which is probably much less valuable to them than yours is to you and put up with increasingly hostile threats[0]. Contrasted with completely ignoring any text that arrives regarding the phone (immediately blocking any arriving from accounts from social media) they're left assuming -- for whatever reason -- you're not even getting the threatening texts.

And, of course, if the scammer were planning on following through with the threats it'd be a very labor intensive, high-risk, low-reward operation -- not the "make a quick buck" that stealing and reselling a high value piece of electronics is supposed to be. There are too many legitimate ways to make more money that involve less work and less risk to pick "crime" as the choice. And while that means it's unlikely the scammer will follow through with the threat, that assumes the scammer is intelligent enough to understand how stupid it would be to expose himself to make a few hundred dollars and tends to vary depending on the amount of illicit substances the scammer is trying to manage the withdrawal symptoms of at that moment. It also assumes that the scammer isn't the actual thief, and the thief isn't some gang thug who'd be more than happy to follow through with the threat simply because "your mean words hurt their feelings"[1].

I figure, ignoring them is the quickest/easiest path to applying at least a tiny bit of pain.

They're gambling; you and every other one of their victims is "the iPhone game". It starts with a box of bricked iPhones. Find the owner, send them a combination of words gleaned from a forum that other scammers have had success with and text your target. You win when you crack the right combination of words that causes a bricked iPhone to become almost instantly convertible into anywhere from a few to several hundred dollars. I'd imagine most slot machines would hit less frequently and pay out less when they do. Enough bricked phones have to become "a few hundred dollars" to feed the player's gambling addiction[2] so you're providing the lever "pull" of the slot machine -- free entertainment for them. By ignoring them, you're taking away that tiny high that comes when they experience "Hope" -- the "hope" that your "pull" will pay out.

Take that away ... block 'em ... but maybe peek into "Spam" every once in a while if you want a little ego satisfaction.

Maybe they sent you a few hundred messages of the "Apple Pay" variety. Now close your eyes and imagine your lone scammer with a box of locked iPhones (it's your fantasy, make him a lonely middle-aged white guy living in Mom's basement who hasn't showered in 2024), checking each bricked phone "to see if you've won", then the messaging app to see if anyone is going to give you a chance to play further ... only to find your fourteenth "Apple Pay" message disappear into the dead Ether ("And they sent the last one from RUSSIA, how could they ignore RUSSIA!?"). If they are the kind with a glass ego, nothing will make you feel less adequate than copying/pasting fourteen unanswered messages as imagine the answer you're not going to get to number fifteen.

Short of maybe social engineering them to click a link to malware[3] and maybe turning the tables a little, I can't think of anything more effective.

[0] And the stones on the thief -- or did your sister leave it unattended? I have spent no time in Austin, specifically, but a lot of time in Texas. Some places it seems everyone is carrying, right out in the open, carefully concealed (if you know where to look) ... whatever. At least a few of my buddies carry hoping for a reason to use it. I suspect in the parts of Texas I'm familiar with, if this happens at all, Find My yields it quickly recovered without police involvement, and maybe the thief's trailer/truck got a little fscked up during "pick-up".

[1] They'll talk about being disrespected but at the end of the day, the problem comes down to an adult who would still fail the most basic parts of Kindergarten (don't misread that statement as implying anything about my feelings regarding the cause/reason/fault of such things; let's just walk away from that one for this footnote).

[2] I couldn't find the study but over a decade ago I recall reading a that people with "clinically diagnosed kleptomania" (naively: "stealing for fun, regardless of need") had extremely strong tendencies to be gambling addicts, but that gambling addiction couldn't be correlated with a higher frequency of kleptomania (gambling addicts do skew higher for theft, but are often thieves out of perceived necessity, limiting to kleptomaniacs was intended to identify the similarities between the underlying psychological disorders).

[3] This is a little tricky and comes with the "you're probably breaking the law, now, too" problem ... tricky because the scammer is probably using a texting app/web service. As a result, there's a larger number of OSes involved -- it would be less unusual for your sender to be texting you via a browser in Windows than you'd probably experience, otherwise, so if you, say, somehow convince them they need to "click that link" in order for your phone to complete unlocking, you're going to also have to work out a way to get them to reveal what OS they're sending from.