Ignore them!!! I mean, just cut them off!!! Most people will say that when they are ignored, they will just go away. Most people are not 'psycho' and will just move on the the next victim. You just have to be watchful of the ones that will track you down and cause you 'problems' in your present relationship. Most 'want to know' from the start that they are the other person and 'understand' when they are cut off for the husband or wife.
This is not the 'best' way to leave a fling... but it is definitely a way....