Change communication channel immediately.
Don't invite someone and when they arrive tell them they are not welcome to speak. Post a list of speakers before, get your feedback and live with your decisions.
This is the best thing that could happen to your event. Use this time wisely to boost your event.
Is it just me or is there a group of people who seem to go to every conference?
Many things were learned. More research was probably warranted. Also, it's not the same to organize a conference for an existing community organized around some technology, e.g. Ruby, Python, NodeJS, etc and organizing an event that tries to span industry topics, it's much harder to know everyone or get references.
The speaker was notified in April 25. The conference will happen in mid-June. She was given fair warning.