I recently had this problem as well, but found that clearing all of the guards out of the region (kill 'em all) before new ones re-spawn worked. (When the new guards spawn, if the old ones are fighting, the new ones will join in, and won't let you pay the fine.)
Kill all of the guards quickly (this takes some strategic planning and adequate combat ability), then find a newly spawned guard and he should ask you to pay your fine.
This is because guards are programmed to help other guards in a fight, and not ask you to pay during a fight repeatedly.
![G o o d [Good]](/emoticons/g_o_o_d.gif)
luck with the mass murder.