can my ex wife ask DHS to no longer be involved in child support collections?

The answer depends on why DHS is involved in your case.  Your ex-wife has every right to close her child support enforcement case.  That will stop DHS if the only reason they are involved is because she opened a child support enforcement case.  However, if your ex-wife ever received any form of public assistance,  such as TANF, Sooner Care, subsidized child care or otherwise, she may have  assigned to DHS the right to recover your share of the costs of such assistance, in which case she will have no control over DHS, at all.   If that is the case, she will also have a duty to turn over to DHS the portion of this "undisclosed sum" you are paying to her that they might be entitled to receive.  She should be able to ask her caseworker whether she can close her case without going into specifics.  Be aware that if she can't close her case, she is required to report all payments through the registry and failure to do so could have severe consequences for her.