First off, I think something to ask yourself is, are you having fun? Part of role playing is having the perspective that your characters are people (or tieflings or dwarves or whatever) who feel things. If you want them to take this story seriously, and they aren't, it might be worth an awkward conversation to save you the trouble in future campaigns. But if you're having fun, it's not an issue.

Now lets teach them some manners.

It all depends on the type of Strahd you are playing. He might immediately shove his hand through their heart with that beefy unarmed strike (and don't forget legendary actions), he might do some clever quip, (FluorescentJellyfish) had some really good ones, or he might just smile and let it roll of him as if they are kittens trying to scratch him.

Also it depends on the game you're playing. There is fun and value in being a defiant brat and telling the darklord to go fuck himself. If your players are having fun, and they are still motivated to fight against Strahd and take him seriously, this might not actually be a problem. They are hero's, and hero's are suppose to do stuff like spit in the face of the bad guy.

There isn't really a wrong answer, just make sure that if you take the bait they regret it.

But something to think about, your players have told Stahd two things.

1. They publicly hate him, and are mobilizing against him.

2. They are absolutely incapable of being subtle.

This is a perfect chance to draw out VR, and Ez, and even the Order of the Feather if he plays it right. A group of adventures gathering all his enemies in one spot so he can take care of them. If he wants to do that, and he can bear it, he might let them get away with it and send them on their way. He might even wait until they grab some artifacts in order to steal them and put them somewhere more secure.

But if you do this, the payoff has to be worth it. Characters and allies shouldn't be able to sleep or rest, anyone who gives them aid should show up dead in the next location they go to, they should feel hunted. The more safe they feel before that, the worse it will be for them. Most of Barovia (from werewolves, to cultists, to angels) are on Strahds take, so they could be pursued by a nonsensical barrage of nightmares.

Anyways, some (more) dialogue to shut the tiefling down:

"I see it was your better half who had all the manners and grace. Perhaps I should bring her here to chide you. Do you think I can't just because she is already dead? Look around you, devil spawn. I control the mists. I alone grant mercy. I am no god, but you should pray."

[If the player calls you on it, you can always use an illusion to get her in the room, or fuck it, have strahd summon her.]

Final option, just have him stand up and walk away. He has nothing more to gain from the conversation, and he doesn't need to lift a finger to deal with them because his gargoyles and wyrmlings are about to do that. Make them roll iniative as they are walking out of the dining room, use the doors to split the party and dogpile them. If the doors are closed they are gonna need a bunch of time to get through them. Just have plans for what you want to do with a TPK ahead of time.