To set a cap on the amount of A/R balance your customers can accrue, you use the Stores Setup menu, locate the A/R Limit field, and enter the amount to which you would like to limit your A/R balances.
Make your desired changes and press Save Store.
AR limit for the individual customer can be set up here (Setup AR Maximum Limit only, Group not required):
Set AR limit to 0.01 to disallow AR on this customer. Note: 0.00 means unlimited.