how to make cat stop pooping under my bed?

First, Keep the cat restricted to one room with their litter box, food and water only. Only let the cat out for short amounts of time when you are home. If any incidents happen, back to the room.
I do not allow my cat into any of our bedrooms and I would suggest that you begin to teach your cat that your room is off limits. When you're not around, keep the door shut. When you are around, anytime the cat goes in your room loudly chase it out of your room. This will establish territory. Both my cats learned this way. I also taught them to use the toilet so we don't have any litter boxes in the house.
You should listen to my advice because I'm a very talented cat trainer. here is a picture of my cat pooping to prove it.

I would just keep the bedroom door closed. You could also try putting a litter pan in your bedroom. Make sure all the litter pans stay clean.

Other than just blocking off access to that area, you probably can't make him/her stop. Once they have decided that is an appropriate place to go, cats are resistant to being "trained" out of it. It's often psychological.

Get rid of it.

hot soapy water mixed with cheap white-vinegar is good for cleaning-up the smelly-spots, plus cats do not like the sharp smell of white vinegar.
what i did was to put multiple cat boxes around, first in places the cat "likes" to defecate, then in places where i want him to defecate. these are usually better screened-away from visual line-of-sight.
to encourage cat box use, i use completely unscented litter, at - once i use 20# of litter for about 6-days before replacing it, scooping often about twice daily, and lastly, to my own surprise, the cats like a super-size cat litter box. combining the large amount of litter, with a big 3.5 ft. x 18 inch x 4-in. deep cat box, really attracts them. when they settle on using the big-box, then i can remove the other boxes, as they have a regular habit of going to one box.
it is like having a super-size bath-room would please a person, but with the same issue, you need to keep it clean and at least, semi-private..

Provide several litter pans in your house. One in the bedroom and one in another place. This way your cat will have access to litter pans.

Scold it.

Throw him outside. There, solved your problem. Thanks for asking.

Keep it out of your bedroom! Also keep it out of any room that you suspect it will poo in! Keep it in the kitchen with a cat litter tray - only let it into your living area when you are around and can moniter the situation.

Block the path under the bed.

get some help

spray lemon

I didn't have a cat. I couldn't give you the answer you wanted. But I suppose you build a nest under your bed, and it should go down to bed.

Put empty boxes under the bed to block off that area.

Close the bedroom door, invest in a covered litter tray

Put kitty litter under there.

its fine for your cat to be in your room, you just need to state that pooping in it is a bad idea. You catch them in the act, beat them. Put them inside of their litterbox. Cats like sandy places to poop, that's what litterboxes are designed for. Restricting your cat from any rooms is a little boring, and you'd hangout in your room most I assume, so, just beat them.