In my most recent adventure, the party was attacking a fort of wooden goblins when the dragon's PC had the brilliant idea of using its fire breath to burn it. I thought it was a smart idea and I was glad he thought it, but I do not trust that this becomes the solution to every problem (especially because an adventure in the near future may involve exploring an evil forest). The reason why I am worried about this is because I think that burning a building from the outside will often be much less interesting than exploring it and fighting its inhabitants.
How can I prevent a player from burning everything to solve problems?
Keep in mind that I'm not particularly worried about knowing if the dragon's breathing is capable of setting something on fire. I do not feel that simply telling the player "the rules say you can not do that" would be a fun solution. I want to keep things fun but also prevent the game from getting out of control.