Rejection of registration to corporate event / conference / convention / summit / congress - not inviting non-sponsor or potential competitor.
Unfortunately, as specified in the invitation and registration form, the event is only open to [specify types of invitees - e.g. clients from the co-hosting firms]. This event is also closed to [specify service providers or competitors not invited - e.g. type of companies, consultants, recruiters etc. and sales, marketing & account executives], unless they are one of the sponsors.