Assault,
She could also just go down and sign off on child support at the child support office.
It's as simple as signing a piece of paper, while the court order would still be there technically she's stating that she no longer wishes for the matenince office to be involved, and they do stay out of it. I signed off for 6? years when my kids dad was going back to school, the only way they would deny this is if she was on welfare (as in that case it's the welfare people getting your money not the family).
Normally, when people do this when the child is still in the home she could come back and get it reinstated (in that case she would have to go back to court), under your circumstances proof that your child is not living at home would stop that dead in it's tracks if she tried to pull something like that.