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Now, let's see what I've got for this one...

First In-game possibility: Combat Experience 'wears off'. Of course, in most systems, there's an experience value given for slaying a certain kind of monster - but it wouldn't be too realistic to award this amount of points each time the PC slays one of them as soon as it becomes routine work... if the brave hero slays his 20th Ancient Giant Multi-Headed Red Dragon, it's surely not an experience as impressive as his _first_ one, so I would also only award about 1/20 of the Dragon's XP value. This forces the players who want to advance to come along with new ways of surviving such encounters - the first Ancient Giant Multi-Headed Red Dragon being _talked_ into submission would be worth as much XP as the first one - maybe even more, depending on how brilliantly it was done.

Second In-game possibility: Involve PCs in Non-combat action. Maybe the Player's favor towards combat comes from the simple fact that they haven't encountered any other things that really are fun to them... WHAT exactly you should do to get your players out of their roleplaying lethargy depends heavily on players and GM. Maybe you could get one of the PCs involved in a love affair? Maybe a few puzzles would do them good? Maybe you could come up with an enemy the players cannot possibly defeat in combat, but only through roleplaying? Depending on the solution, it could even be helpful to confront them with an _extremely_ boring combat situation short before you come up with The Other Possibility. Sometimes contrast does a lot.