How do I stop/deny a user from re-visiting a submitted form

I have a quiz that I would like to use multiple times for different groups.

However, I cannot stop a user who has previously submitted a response from re-visiting that form and copying/sharing the questions and answers with other users.

The only way appears to be by deleting all previous responses, but that stops me from collating data for review at the end of the year.

Thoughts?

PS I’m on UK local time.