Option 1 - disable account
This is the best option if you may need to access the email history of the user.
NOTE: This option will remove all access to this user account; however the account will continue to receive email and consume a licence
Option 2 - delete account
This is the best option if you no longer need to access the email history of the user; however you can still elect to setup any email aliases with another user
NOTE: if the email is not backed up, all information will be lost. Depending on the size of the mailbox being backed up, additional storage may need to be purchased.
Option 3 - reassign account
This is the best option if you would like to access the email history now as a shared mailbox or have a new user take over this account and all email history