Hecklers have one goal: they want to get you to overreact, make a fool of yourself, and break the rhythm of your speech. Let them spew their venom for a few moments and then tell them that you can address their concerns when you take questions later, along with those from everyone else. That will also buy time for you to think of how you want to respond during Q&A. If the heckler stops at that point, when you come to the end of your talk, you can rephrase his question and answer it in a way that suits you. If he does not accept that answer, you can give him one follow-up question to let him get it off his chest, answer it as best you can, then tell him that others in the audience deserve to have a chance to ask questions, and that you will be happy to talk with him afterward.

If the heckler continues to actually interrupt the speech and there is no host to intervene, ask the audience to raise their hands if they would rather have you finish your speech, instead of listening to him for the rest of the period. That should do the trick. If the heckler appears to be drunk, give him one warning that he will have to behave himself or you will call security.