Your web developer can hide the duration field from the Booking Form using CSS:
> If none of the booking types that are going to be appearing on the form requires a duration then your web developer can change the style of .dmn-duration-container with their CSS.
> If you want to just hide it when it's disabled then your web developer can do this changing the style of .dmn-duration-container.disable
If you are removing the Duration field so that customers can not select this themselves, there MUST be a default Duration set up in your settings (See guide on how to setup 'Set' durations) or you can Remove Duration so that the customer can stay until your closing hour.
Do pass on this information to your web developer.
More details on this can be found here.