user leaves company

Scenario: I have 200 users who have contibuted to force.com for 5 years.  When 1 user leaves the company I will migrate the “to-do’s” to a new user.  However, I want to still see the notes were associated with the separated user. 

10 years from now when someone is looking at the note they can see who made the note 10 years ago?

 Is there a method to keep the user in force.com, and deactive their license.  

Deactivating licenses is the reccomended strategy.  The user does not have access to your system,  his license does not count toward any contractual limits, and you keep any data tied to the appropriate party.