How to restore form from trash and purge submissions?

Will not let restore form from trash to purge submissions.

I am not sure if I have understood your question correctly. 

Please note that when you delete your form, it is just moved to trash folder and no submission data is deleted. When you restore the deleted form, it will restore the form along with the submission data.

The only option is to delete the submissions manually or purge the form itself from trash folder. 

Hope this helps.

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