+Sean McCrohan "If I invite you to speak at a conference, then withdraw your invitation because people have expressed disagreement with your position to me, has your free speech been violated? "
Let's assume that the organisers had a good reason to invite you in the first place. Then let's assume that the protesters expressed more than disagreement. They threatened adverse publicity, rallied other supporters to their cause and threatened boycotts.
In this case the organisers would cancel out of simple necessity. They have little choice in the matter and one could not accuse them of silencing free speech.
On the other hand, the protesters had several choices.
1) they could have stayed away from the conference,
2) they need not have attended the talk,
3) they could have answered the undesired free speech,
4) they go the whole hog, rally supporters and force cancellation of the talk.
In the case of options 1 to 3 they are making free choices that affect primarily themselves. That is their right.
In the case of option 4 they are trying to impose their will on
1) the speaker,
2) the organisers,
3) the other attendees.
This is both silencing and the denial of the moral right to free speech. There was no necessity to their behaviour, they had lesser options if the speech was indeed repugnant to them.
Most importantly, their actions are a denial of the important principle that undesired speech should be answered with better speech so that the community at large can hear both sides and make a more informed decision.
Put very simply, what right do you have to decide what speech I should hear? Silencing is your attempt at denying me the right to hear other people's speech. It is plainly and simply indefensible.