How to stop the smearing

After this many years, I really can't take anymore. Please, can someone give me a suggestion that they saw definitely work with someone who cannot let go of this stim? Yes, I am the parent. Yes, I love my child. This does, however, seriously need to stop.

I went through this for years myself with my 7 year old son... the only thing that helped us was the suggestion of overalls... bib overalls... a mother who has several kids on the spectrum suggested this to me. what she (and myself) did was find overalls that fit the child very well. sew up the sides where the buttons are so that the buttons cannot be undone, and you put the overalls on backwards and twist the straps so they cannot be pulled off the shoulders.... and then they cannot reach their poo to smear it.... worked for us, until Emmett and his 6 year old little brother teamed up and undid the straps for each other.... lol