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 by changing the style of .dmn-duration-container.disable.
⚠️Warning: If you remove the Duration field so that customers cannot select this themselves, there must be a default Duration set up in your settings, or you can remove duration, so that the customer can stay until your closing hour.