Spontaneous wage protest: Can we fire the agitator who stirred up his co-workers?

Q. We are a nonunion company with a call center employee who has been stirring things up. Recently, he and a large group of first-shift employees stayed in the parking lot instead of coming back from lunch on time. A few of them held up signs saying, “Fair Wages Now!” We’d like to fire the bad apple. Are we taking any big risks if we do?