Really there are two elements which define the hack&slash vs. roleplaying quotient in a campaign: 1) The cues and motivations provided by the GM, and 2) The preference of the players. With enough practice, any GM can get the first part right, but there is really only so much control one can have over the second. It is entirely possible to have some players you simply cannot ween from combat happiness. Some people would suggest that you find another group when faced with this situation, but I think there are some things you should try first. These tips are here to help you make combat more fun for you. Obviously, if you have a complaint about the level of hack&slash in your campaign, then there's something about the hack&slash you don't like. If you want more roleplaying in your game, then maybe you should try using combat as a vehicle for that by:
• Having your villains talk trash. Far too many GMs overlook this simple and fun way of spicing up a melee. Best of all, characters (or players) may feel a need to answer your obnoxious antagonist in kind, and before you know it, you're roleplaying! One of my party's favorite encounters was with an orcish highwayman that I had talk to them like a pro wrestler; every round of combat, he delivered a new macho "witticism" that I prepared beforehand. They had a ball, and so did I.
• Using interesting scenery. What's more interesting? Having a mass melee on a forest path, or having it on a rickety rope bridge overlooking a 100 ft. deep chasm? If nothing else, the latter keeps the players on their toes.
• Injecting a healthy dose of moral ambiguity. Have the party befriend two NPCs at different times, and then have the two NPCs meet in front of the party as mortal enemies. Make sure to encourage your PCs to choose sides --but don't make them too comfortable with it. If the players become frustrated or emotional, then you've succeeded in immersing them in your game world.
• Having a "combat", *wink* *wink*, *nudge* *nudge*. Certainly not something you should try on a regular basis, but bring in a character you want to have interact with the party as a combatant (like a sensai, or sympathetic villain), and fudge the rolls so a whole lot of nothing happens. Give the PCs some experience or skill points for the encounter (especially if they roleplay appropriately), but let the NPC get away.
• Being horribly dramatic. This is what you're going for, anyway. Give the dying villains final words; describe important hits and misses in great detail; create extra consequences for failure; have the antagonist give an impassioned speech before drawing his sword -- whatever floats your boat. Remember, if you think it would be cool, your players will at the very least find it memorable.