My client has also requested for this feature. 

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I'm trying to work on process improvements to manage staff signing up for seminars then not turning up.

I know I can change settings to prevent them from signing up again or to limit how often they can sign up - but if I put such a rule in place (eg users can only signup once), is there any way that I can enrol a user overruling this?

Alternatively, is there any way that a user with a previously recorded no-show would require my approval to enrol?