First off, it's as much their game as yours, so if they're really having fun hacking and slashing, i'd say that needs to be a big part of the campaign for the group.
1)On the other hand, can you level with your group, discuss with the a major part of the game they're missing out on that you want them to look at.
2) Start including some challenges that can't be killed. Puzzle traps, mazes, you get the idea.
3) Start rewarding, players for role-playing, announce straight xp awards at the end of sessions to the best roleplayer, have in-game results (good and bad) according to actions they take (if they turn over the bandit chief, he'll end up reforming and giving them x info/stuff, if they just brutally slaughter everyone, more enemies are formed or something).
4) include cursed items, some of my current party members were a bit to hasty in looting some items, and now labor under a creative (and powerful) curse
hopefully this all helps. good luck