Once your Ignite gets more popular you'll probably find that you have more applicants than speaking slots. It's never nice telling people that they've been unsuccessful - here is an email you could use to let them down gently:
Hi there,
Thanks so much for submitting a proposal to speak at Ignite XYZ. We've been overwhelmed by the number and the quality of all of the submissions.
Unfortunately, there has to be a cutoff point and we can't showcase all of the ideas submitted, so we can't go ahead with your talk at Ignite XYZ #123.
Thanks again for your contribution, and we hope that you can still come along on the night - there's a ticket in your name on the guest list by way of saying thanks.
The Ignite XYZ team
This work is licensed under a Creative Commons Attribution-ShareAlike 4.0 International License.