You can unassign a checklist from an employee on the Onboarding page in the Onboarding system. Locate the employee who has an unnecessary checklist and click the 'Assign Tasks' link next to their name.

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Uncheck the box of the checklist you’d like to unassign from the employee and click the Update Checklist Assignments button at the bottom of the page.
 

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If you're an admin on the Onboarding side, you can also remove an individual task from an employee's checklist if you'd rather simply remove certain tasks from this checklist. More info: How do I remove a task from an employee?