In USA, all programmed must submit to the IRS when they pay contractors (in this case loyalty members) $600 or more during a tax year.
Horizon tracks what a member redeems and logs the perceived value. This value perceived value is added to total spend for the current tax year. When the value goes over the $600, a redemption will be rejected. For example, if their tax year value was at $585 and they try to redeem an offer at a perceived value of $20, the $20 offer would be rejected.
When a member hits the $600 threshold, the member's contact record will show this.
In the member's contact record, go to the 'Tax level checking and monitoring' section. You'll see a 'Latest rejected redemption' box', which will tell you the last redemption that failed.
The member will need to be informed that they have reached their limit and they must submit a tax form. Once the tax form has been received and approved, you can update their member's account.
Click 
within the 'Tax level checking and monitoring' section. The approval will be added to the 'Tax form approval history' section and the member can redeem again.
You can also keep track of a member's rewards perceived value for the tax year. You'll see the year and total amount under the 'Tax form approval history' section.