First, never delete an ex-user as a user. Simply disable their login to free up their license.  Deleting a user will force you to either delete all of the data they created or reassign their data to a different user.  Neither of these are good choices for most companies. Most companies always want to retain the ownership of a record.

An ex-user's calendar can be handled to accommodate your needs.  Keep in mind, when an activity is "cleared" as Completed or Not Completed, it is committed to the history for the respective contact(s).

If the activity is erased or left uncleared, it is not committed to the history.

You can clear multiple activities at one time from within the Task List. Filter those activities you want to clear or erase. Activities can be filtered by User, Date Range, Type, or Priority.  Highlight (click) the first record and then hold down the Shift key and highlight the last record. Right click on the highlighted tasks and choose Clear Multiple Activities.  Decide how these activities are to be committed to history OR you can erase them.  Keep in mind, the person who is logged in and clearing the activities will have their name next to the history.  If you want the histories to include the ex-user's name, you will have to login as the ex-user and clear the activities.

ACT! does not have a feature to reassign activities to another user. A workaround is to login as the user who should be assigned the new activities.  Next, have that user filter the Task list for the ex-user's activities. Highlight them all and choose Edit > Cut from the ACT! menu.  Wait until the activities disappear.  Choose Edit > Paste to put the activities back.  This will cause the activity to be put back into the database but assigned to the user who is logged in and doing the cut and paste.

Caution, the more activities you cut, the longer it takes to see them removed from the screen.  Paste will also take a longer time when there are large numbers of activities that have been cut. We recommend no more than 100 at a time.