Enabling credit at the advertiser level can be done within the advertiser's window.
From the Main Menu, Go to Advertisers -> Advertisers. Click on the Advertiser's name to open the Advertiser Window.

From the Advertiser's window toolbar. Click on Advertiser Credit -> Disable Credit...

You will need to confirm that you're disabling credit at the advertiser level.

Notes:
- Disabling credit does not mean the advertiser does not have credit to purchase leads or calls. It means, the advertiser credit is not enabled.
- Disabling credit will disable all contract's credit using the advertiser credit.
- Disabling credit will not issue refunds to the advertisers' credit card.