You need to find out why your player(s) want to resolve conflicts non-violently, and in particular you need to figure out whether this is a whole-group issue or a one-person issue. (It's not clear from your question whether a single player is responsible for all the attempts at talking, or if most/all the players do it in turns.)
If a single player is always trying the diplomatic route and getting upset when it doesn't work, then talk to him outside the game, where he won't feel pressured. Ask why he always goes for the diplomatic solution. Perhaps that's his preferred playstyle, or perhaps he built a diplomancer character and feels like talking is the only way he can participate in boss battles. Once you understand his concerns, work with the player to alleviate them. Perhaps give him a chance to roll up a new character with a more martial focus, or find ways to incorporate diplomatic challenges outside boss encounters where his diplomancer can shine.
On the other hand, if the entire group is more interested in talking than fighting, then you've got a playstyle mismatch. Take some time to get yourself and your group on the same page, using the Tool or otherwise. This may mean you modify your Traditionally Serious Campaign into something with more intrigue and interpersonal relationships; or it may mean your players reroll characters or simply alter their own playstyles to fit the campaign you're running. Which you choose is up to you and your group, based on what works best and is most fun for all of you.
A third, middle-ground option would be to give the diplomancer player(s) what they want! Not all the time, but once or twice, enough that they feel they're getting a chance to do what they want. Plan at least one boss for whom a diplomatic solution would make sense. Use this to compare and contrast the traits that make for a talking encounter, with those that make for a fighting encounter. I've had several bosses designed in such a way that they could either be battled or talked down, and let the players choose which they prefer. Done right, this can lead to some incredibly dramatic, interesting, and fun roleplaying opportunities, such as the time one of my players almost talked down a villain (complete with Cooldown Hug!) - only for something to go wrong at the last minute, when the villain betrayed the character and nearly killed him. This drove a huge roleplay moment for the PC, and made a later confrontation with that villain much more poignant (and the earlier confrontation meant that in that later one, they had a much better chance of actually talking the villain down). In a different campaign, we had an ongoing villainous dragon whom the party did successfully bargain with, despite the assassin's, ah, intense desire to the contrary. Again, a tense RP moment for the group and a fun character moment for the assassin's player.
TL;DR: Find out why your player(s) want to talk instead of fight, then find ways to adjust your own or their expectations to make boss encounters more fun for everyone.
