Options...
Doc -
As a long-time reader of your story hour, I know your PCs don't get mad, they get even
!
The dragon is royally pissed, but it also has a dragon-sized ego. Killing the imbiciles that dared to come into its home is far too easy - it would want to torment and humilate them! That said, I think you have some interesting options here:
(1) An immediate counter-attack should be in order, before the PCs have time to regain their spells and rest up. Have the dragon swoop in (or tunnel up under the inn) and grab a PC or two (preferrably the ones that can teleport). After knocking them out, he strips them naked and sets them free up in the mountains - telling them that it will brook no further intrusions and commanding that they inform their fellows, if they live.
(2) The dragon returns to the village and takes some hostages to ensure the "good behavior" of the PCs. If the PCs keep messing with the dragon, he turns the hostages into "villagersicles" (impaled and frozen) for a late-night snack.
(3) Since they teleported into his lair and were flying last time, the dragon would definitely take some precautionary measures. A fine one would be to line the roof of the main cavern with huge icicles. A massive tail slap as the dragon retreats into its alcove brings the icicles raining down on the PCs, causing crushing and cold damage.
(4) Blackmail - The dragon utterly wipes out a village in a nearby kingdom, then sends the rulers a message..."Sit on the obnoxious adventurers or else. Every time they bother me, I will wipe out another village. If they leave me alone, I will leave you alone!"
I would definitely take every opportunity to ridicule, torment and bitchslap the PCs with the dragon - not to kill them, but to show them that this wyrm won't be toyed with.
It should make their eventual triumph over the beast that much more delicious if they really hate it!
~ Old One