Kazutaka Kurihara & Koji Tsukada, "SpeechJammer: A System Utilizing Artificial Speech Disturbance with Delayed Auditory Feedback", arXiv:1202.6106v1 [cs.HC], 2/28/2012:
In this paper we report on a system, "SpeechJammer", which can be used to disturb people's speech. In general, human speech is jammed by giving back to the speakers their own utterances at a delay of a few hundred milliseconds. This effect can disturb people without any physical discomfort, and disappears immediately by stop speaking. Furthermore, this effect does not involve anyone but the speaker. We utilize this phenomenon and implemented two prototype versions by combining a direction-sensitive microphone and a direction-sensitive speaker, enabling the speech of a specific person to be disturbed. We discuss practical application scenarios of the system, such as facilitating and controlling discussions. Finally, we argue what system parameters should be examined in detail in future formal studies based on the lessons learned from our preliminary study.
Bernard Lee wrote in "Some Effects of Side-Tone Delay", Journal of the Acoustical Society of America 22(5): 639, 1950:
By plugging a telephone headset into the playback jack, a person's voice may be returned to his own ears but delayed by the length of time the tape requires to move from one magnet to the next. The effect of this delayed and dominating echo is startling — it will cause the person to stutter, slow down while raising his voice in pitch or volume, or stop completely.
Since 1950, we've shed the encumberances of mere analog matter (moving magnetic tape, recording and playback heads, a telephone headset etc.), and can achieve the same effect using a simple digital delay along with directional microphones and speakers. There were once librarians who could do this with a frown and a gesture, and perhaps in the future the preferred technique will involve hacking into the offender's cortical implants; but for now, this is the state of the speech-suppression art. Though perhaps use of the SpeechJammer counts as assault or some other legal offense, I'm not sure…
For background on delayed auditory feedback, see "This delayed and dominating echo", 2/14/2010.
Update — I should add that shorter-delay auditory feedback has been claimed to reduce stuttering symptoms. For a review, see e.g. Elena Antipova et al., "Effects of altered auditory feedback (AAF) on stuttering frequency during monologue speech production", Journal of Fluency Disorders 33 (2008); or Michelle Lincoln et al., "An Experimental Investigation of the Effect of Altered Auditory Feedback on the Conversational Speech of Adults Who Stutter", Journal of Speech, Language and Hearing Research, 53(5) 2010.
[Hat tip to Cynthia Hagstrom]