Hi
I also work with a young adult with severe Autism whom smeres feces as well. I am not sure of the age range that your speaking with or what type of setting that your in. We are a community residential setting. The guys that we work with have access to helping us do chores even if they are prompted or hand over hand. When our guy smeres we have him help clean it up hand over hand. It is not pleasent for neither. He hates the gloves he has to wear and also hates to help. Then we get him to put the soiled clothes towels etc in the laundry. But, within a couple weeks he is no longer smereing. We do still get the odd time. But if this is a school setting it would be not a great place to work on the behaviour.
When he was in school and he did this we would do a round of sensory activities before toileting him. This seemed to help a bit. We made up a bunch of containers with lids. In the containers we a variety of different scents. Anything from coffee to vanilla to materials to spices. This seems to satisfy the need to smell anything innapropiate.
This also worked with my own two children who have Autism.
It would not be a bad idea to provide alternative activites that provide sensory stimulation, but these activities should not be connected to fecal smearing attempts. That is, if you want to increase stimulation, do that according to a schedule independent of fecal smearing. Some sensory programs inadvertently reward inappropriate behavior. For example, some programs involve allowing children access to a swing when they are behaving inappropriately in the classroom. In this case, the child may learn that they get a break and a fun activity when they behavior inappropriately.
I would strongly encourage you to focus on close supervision during toileting visits and other opportunities to smear feces. Any attempts to manipulate feces should be prevented by gently guiding the child's hands away from the area. Do not comment. If close supervision is not possible during all times when fecal smearing is likely, you might try a reward system. At first, give small frequent rewards for keeping the area and hands clean. You can later reduce the frequency of rewards. Also, the child's pediatrician should rule out (or treat) any medical reason for this behavior.