NOTE: While not recommended you are able to disable the tax calculation within ClockOn, It will not display the employee Tax category on certain reports and payslips.

1. Open Employee details,  Navigate to the employee and select the "Pay" Tab.

2. Set the Tax category to "No Tax" As shown below. 

3. Then click Edit button next to allowance set to enter the allowance screen you can then enter an additional tax allowance for the desired tax amount. (As shown below)

This amount can also be edited at payroll.