If this is about the advance (monthly) child tax credit payments, the answer is the same: you cannot stop somebody else from claiming your child, to get it. You can use the IRS tools to make a competing claim. Ideally, the other other party uses the toll to stop the payments from coming to them. If you are unable get the monthly payments, then you'll have to wait to get all the child tax credit (CTC), when you file your tax return 2021 tax return, in early 2022. The other party will have to pay it back, if you are the one entitled to the CTC.
See previous answers about competing tax returns.
