In Arkansas the child support terminates by operation of law (meaning you do not have to do anything to make it terminate) when the child turns 18 years or should have graduated from high school, whichever is later.
If you have a income withholding order for your child support, Arkansas provides that if you do not owe any arrears you may terminate the withholding by sending a notice by certified mail to your employer, the custodial parent, OCSE, the Clerk of the Cort and the Child Support Clearinghouse. You may also want to include an affidavit from yourself, the custodial parent and OCSE setting forth that the child has reached the age of 18 and has graduated from high school and that you do not owe any arrears.
Note, however, that some employers will require a court order before they will stop withholding the support from your wages. In that case, you will need to file a motion with the Court to request it to enter the order