Bed soiling in autistic adult.

Hi, i work with an adult who  has learning disabilities and is on the autistic spectrum. in the morning when daytime staff come into her residential home she soils the bed and with some staff she does it throughout the day as well. She knows how to use a toilet and even has a commode in her room she just doesn't use it but uses it when asked to or when she wants to. We have tried various approaches at stopping this but these have not worked. We believe that she does it mostly because it gives her some control and have tried introducing more control into her life but this has not worked. does anyone have any suggestions?