You can change the owner of a task by completing the following process:
Open Access TaskCentre and navigate to the Tasks folder
Double click the task to open it
Select the General tab
On this tab there will be a drop down for Owner
Select the drop down and set the new owner for the task
Select OK
Repeat steps 2 - 6 for all relevant tasks.
Once the owner of the tasks has been changed, you can then remove the relevant user from the system using the following process:
Select System, Users
Right-Click the user and select Delete