Visit the Hiring Plan section of your account from the user icon menu to reveal the requisition fields. Find the field/s you wish to disable and toggle them off so that they are not used when creating new requisitions. Any field that you disable will affect all future requisition forms and not existing reqs.
Requisition fields already used in requisitions cannot be deleted. If you added a new field by mistake that hasn't been used yet, you can delete it by clicking the three dots to the right of the field.
Note that default fields cannot be deleted, but some of them may be disabled by toggling off the switch to the left of the field.