Well, actually, in some cases which have now passed constitutional muster, you are. Some municipalities and counties have established 4th degree assault, which amounts to causing undue annoyance. You may tell me something once, politely. You may not pursue me down the street exercising your freedom of speech in my vicinity for a half-hour.
There's a limit to the approach or delivery, I agree. That's where tpaine was going as well. That's independent of the content. A sales-clerk pursuing me down the street for half an hour repeating the message "Thank you, please come again!", would be a menace or a nuisance.