You have the option with most Authorization Definitions to use the single-use setting. You choose to enable this option for a definition simply by selecting the shown check box in the Authorization Definition Dialog:

Most definitions enable this option by default. If you want individual generated Activation Codes to be used repeatedly, de-select the "Single-Use" check box. Remember, if you choose to make a definition single-use, it does not mean that a specific user can only ever Activate that Code once - it just means that have to get a new one from you every time they wish to do so.