how should you react to someone who causes 'offence' who wants to start a confrontation with you

in real life? someone who says things to trigger and provoke you into a confrontation, because that's what they want, and all you want is to not get involved or lower yourself to their level, because if you did it would be too bad for them and it's you who gets in trouble.....i've encountered people out there like that before and it was very hard to keep my cool.