No, you cannot delete a user who performed tasks in Altru. Instead, you should mark old users as inactive to retain a history of their actions.
  1. Click Administration > Application Users
  2. Click the arrow next to the username
  3. click Mark Inactive

Note: Are inactive users cluttering up your Application Users page? You can hide them! From the Status column, click the funnel icon. Select all check boxes, but leave Inactive clear. Click Apply. Only users who have a pending invitation status or are active appear.