Usually known as “trolls” on mailing lists. Hecklers are self-centered and irritating to everyone--especially the audience. Do not let a heckler take over YOUR presentation. If they were any good they'd be at the front instead of you.
ask for people to hold their questions until the end of your presentation
tell the person if they are being disruptive (“I am finding it very difficult to concentrate on my presentation” or perhaps “It is difficult for me to speak over you--it is making me tired having to speak at this volume.”)
ask the person what their name is. Refer to them by name whenever they interrupt you. (“Matt, that's really interesting, but not part of my presentation.”)