Yes you can, but there are certain conditions that have to be met which we've put in place to prevent abuse (like somebody trying to "cover their tracks" if the form is an evaluation form of some kind).
Except in very rare cases, all online forms should be season / event-specific so that the responses are grouped by season / event. Since it's easy to copy from a previous form when creating a new one, it's worth the tiny effort to create new forms each season / event, which then also makes it possible & easy to delete an old form and its responses from a previous season / event.
A form can be deleted if it has no responses, or if all of the following are true:
- The form has an end date specified
- The end date was at least 8 months ago
- The last response from the form was submitted at least 8 months ago
- The form has not been modified in the past 2 months
So what that means, is that if an old form didn't originally have an end date specified but hasn't had a response in the past 8 months - then the form would need to be updated to add the end date (of at least 8 months prior) and then after making the change there is a 2 month waiting period before being able to delete it entirely.
Those rules try to strike a balance between preventing abuse and allowing a bunch of submissions to be legitimately deleted. Because we don't know what the forms are being used for, this is the safest approach.
Also of note, that if a form and all its responses can be deleted, that this will not trigger any email notifications (unlike when an individual response is deleted).
Hope that helps,