I had a user who was having the same issue. The only way I was able to disable the forwarding was to create a Custom Communicate - User role with no call forwarding permission. I assigned this role to her and took away the original Communicate - User role. This stopped the forwarding. Scared to re-enable it, because the last time I did, initially everything worked normally. Then, after a little while, without any warning, her phone completely stopped working. I got it to work again by deleting the number from her profile and re-adding it, and then it started forwarding again! At that point I had to give her the modified Communicate - User role again.
Hoping that we can get an answer on this one as I do want to re-enable call forwarding for her at some point.