Prodigal 2 and I have minor pseudo karmic curses, reserved for people whose behaviour range from mildly annoying to needing a thump. For the minor offences, like blocking an aisle in the supermarket, the curse might be something like this - "May your foot find a random bit of lego, specifically at three in the morning, while you are on the way to the loo." If they have been rude too, it may change to - "May your foot instead find an upward-facing three pinned plug". Or, if they have been a bit cheeky in the traffic queue - "May every light be red on the way to your destination."
We have also got pseudo karmic blessings which can be used - if someone has let you go in front of them in a queue, because you have a basket and they have a trolley then - " May you find a tenner in your spare jacket in the back of the cupboard", or "May you find the last parking space available, with adequate room to manoeuvre" would suffice.
The key is to be mild in your manner and inventive with your curses/blessings. Failing that, I have found that one can utter the phrase "Go die in a hole" without moving one's lips.
Probably not in the supermarket, though...