Would "cut and paste" work for you?
The same phrase, over and over?
The only *acceptable thing* for them to actually be saying is, "wow, you're still not back to full function? What a bummer!" (for *some* of them, that may be what they mean; for the others, you can sort of politely "coach" them)
So pretend that's what they said.
Say, in the tone that implies you're agreeing with them: "Yes, it's still healing. Bummer, huh?"
Or, "Yeah, it is taking a really long time."
And if you pick one that feels natural for you, then just stick with it with every single customer. Pretty soon they'll figure out that they never get any response, and maybe they'll fade out on the commenting.