To manage the MasterCards on your account, select "Cards" from the menu along the left, then "Manage Cards from the sub-menu.
You'll be able to select a card from the list, use the search menu, or filter based on status or card type. You can use the search menu to search by Name or the Last 4 digits of the card number.
Card Status: Active/Blocked
From this main page of manage cards, you can choose to activate or block a card from the list by clicking the status button alongside the card you've searched for.
Active cards will have a green light, and those that are already blocked will be gray.
You will be prompted with a screen to verify when either blocking or activating a card from the list.
Card Controls/Spend Limits
Card controls allow transactions based on merchant categories. For example, if you don't want the card to be opened for every merchant possible, you may select Travel & Expenses to allow use for Travel, Lodging, Food, and Fuel. This can be edited at any time.
Spend Limits control the dollar amount accessible on each card. For example, if you want a coworker to have up to $50/day you can set that. You can edit this to reset on a daily, weekly, or monthly frequency.