I've had clients call asking about opting out so I looked into what is required.  You could assist a client but there is no way to do it for them.   I began setting up an IRS account for me to see what all it entailed.  Client must have an IRS account which requires an email, email verification process, receiving a code by phone for second verification, uploading a driver's license or state id......... that's where I stopped.   This was just to get the IRS account - not sure what would required to actually opt out of ACTC.