Hi jbruyet,
Deleting a user and transferring tickets before deletion, works a bit odd. Before deleting a user/agent, all data associated with that user/agent needs to be transferred to another user/agent.
You can do this under Configuration\User Access & Roles and click on the transfer option at the right-hand side of the users' list. Make sure to select the button next to the user to whom you want to transfer the information to. From the popup window that appears, select the user from which you want to transfer the data from.
Once transferred, the user can then be deleted.