Wrap the cat firmly in a towel so it looks like a sausage roll with its bum hanging out so you do not get slashed to death. If it is fresh (i.e. wet and soft) poo wipe off with baby wipe or wet kitchen roll, it is is hard dried poo, clip it out with scissors.
That sounded a good idea, I got it in the towel but it was impossible to get just it's bum showing, when I got to work the cat went beserk, contorting wildly, it's back legs, claws out, going like a wind mill, face spitting and frothing like the girl in The Exorcist, it was scary. It got one front paw out and things really went downhill fast. I've got some poo off, it's on the towel now.
It shot outside, hopefully for a long time, I have been thinking, if it comes back and the poo is dry, then I have some hair clippers that I used to use on my sons when they were young.............number 6 might do it.