Operations are typically automatically closed. This option is basically just used when overriding the normal close. I.e., closing an operation before full quantity is reported, or keeping a fully reported operation open.

You can anyway try to configure this assistant using page designer to hide this setting for the operator. 

Setting up dummy work guidelines , adding a competency profile for signoff , signoff required to close operation options can be used to restrict employees from closing operations.

But it will result in some additional work to sign off the dummy work guidelines.