I'm trying to figure out why a new thread is an effective way to handle this. Your ads (a) specifically direct prospective purchasers to text you, and (b) aren't, as far as I saw, marked as 'sold' either in the header or in a reply. Somewhat famously, there isn't a time period after which all readers presume that an animal is sold.
There also (and also famously) are thousands of scammers and thieves trolling ad boards like this one looking for emails and phone numbers to reach out to in order to take advantage of someone. They know low-hanging fruit when they see someone unwise enough not to use the PM system for sales contacts. Likely at least some of your annoyers are professional scammers. Lesson learned, hopefully, to keep the place clean so as not to attract cockroaches.
But for the legit buyers maybe a polite reply to each of the ad threads pointing out that the animal is sold is in order? Sounds too simple to help, I know, but has the virtue of being pretty easy to figure out.