Imagine the memory, turn it into a photo printed on a piece of paper. Drain the colour from it turning it black and white. Turn down the picture quality making it grainy. Now in your mind's eye push that piece of paper as far away from you as possible so it's just a dot in the distance. Let it get blown around by the wind, to left and right, up and down until the piece of paper is battered and torn. Then mentally throw that paper in the bin.

You can do this with all unpleasant memories which keep reappearing.