Form+ provides an option to set a submission deadline for your forms. To do this:
From your forms page, click on the Settings option for the form you would like to set a submission deadline for or navigate to the Settings tab in the form builder after creating your form.
On the Basic tab (default opened tab), there is a Submission Deadline field/option, select the date you would like to stop receiving submissions for your form and the click on the Save Options button
Please note that the form will stop receiving submissions from 11:59 pm of the set date based on your time zone.