If you delete an agent account, all the tickets associated with that agent will be moved to "unassigned." To avoid this, kindly give that agent account a dummy email address inside the agent's profile by clicking on edit next to the name and entering this in the email tab. Once this is done, please make this an "occasional agent" account.
This way, you won't be charged for that account, the outgoing agent won't be able to login to your system and you'll also continue to have all the data relevant to that agent.