How could you effectively protest a speaker and not interfere with the ability of the speaker to speak?
Here are some ideas:
You get there early with large group of people, all wearing bowler hats.* Your group fills the front rows. Once the speaker is introduced, you all begin very quietly to sing "We Shall Overcome." Quietly singing, you file out of the auditorium one row at at time. The speaker completes his racist speech after the first 10 rows have been emptied. You have interfered a bit by delaying the speech and singing very quietly, but your opposition is symbolic. If you had enough in your group to fill the entire auditorium, so much the better.
You get there early with your group, fill as much of the auditorium as possible. After the the speaker is introduced, you applaud sarcastically for 5-10 minutes, shouting words of praise. Then you all take out your headphones, take off your bowlers, and listen to music while studying from your textbooks. When the speaker is done, you take off your headphones and applaud for another 10 minutes, delaying the Q and A for ten minutes. You ask all your questions in a foreign language. Or do them as knock-knock jokes or sarcastic questions: "What, in your opinion, is the most effective way of keeping black people in their place." Hand these out to your group in advance.
If it is in a room with chairs not attached to the floor, turn them to face the back of the room.
Fill the front rows with couples kissing, both heterosexual and not.
Have some jugglers at the back of the auditorium doing tricks very quietly.
The idea is the following: protest with some amount of wit. You are not depriving the speaker of the right to speak, and someone who is there to listen will still be able to listen. You are not obliged to listen, or to listen seriously.
Or, you could just listen and then in the Q and A put forward your best debaters.
*Why Bowler hats? Because this is a surrealist protest; your are channeling bowler-clad men from René Magritte.