Rather than 'un-publishing' surveys at their completion, click the "Close Project to Responses" button. This will keep the data that has been collected for reporting purposes, but not allow anyone else to provide responses to the survey questions.

This will close the project to responses, but leave the results visible to the community.

​Related articles:
​How to un-archive a project re-open a survey
​How to set a close date for receiving input
​Deleting and archiving a survey
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