Since it sounds like you cannot access the space, you will have to find where the animal(s) is accessing the space, then determine the best way to capture the animal (e.g., live or lethal trap), then close any and all openings you've identified as ways the animal was able to enter the attic space. If you can figure out where the animal is entering the space, set the trap near this area to increase chances of capturing your target animal. Again, do not close off the space until you're sure you've captured all the animals inside.

I didn't provide a link to any lethal traps as these are best left to professional trappers. Do not use poisons. If you do not feel comfortable live-trapping and euthanizing or relocating animals, contact a local wildlife removal service.

Best,