Can I ask for an accounting of how support is spent or dictate how support is spent?

No.  The support is paid to the other parent and used as that party sees fit.  Often I hear that “none of that money goes to my kids” and indeed, that may be the case.  Remember that support is paid to pay for the child’s individual needs (ie clothing, haircuts, shoes, coats, etc.) but also meant to cover a portion of the child’s share of all expenses in the household (ie utilities, groceries, rent/mortgage, supplies, heat, etc.).