Its pretty easy. You have to be a frost specced mage with Ice Block (2.3.2 any mage will have IceBlock.) Flag yourself and duel someone. When they are in mid attack or spellcast use a macro that is like:
/cast Iceblock
/forfeit
They will finish their spell hitting you then the guards will kill them.
Also, if you see a flagged player in a neutral city. Cast a spell on them to have them enter combat with you. (fire blast works best) so they enter combat and Iceblock, the guards will kill them and forget about you.