If you go on one or two dates and you know you are not interested, how do you let them know? Ignoring? Texting? Calling? What?!?
Over the course of your dating lifetime, it's likely that you'll either be on the giving or receiving end of all of these unfortunate ways of ending a relationship before it's even gotten started. Really, there is no great way to tell someone you're not interested. Don't even think of saying, "It's not you, it's me." Sex and the City taught us that you better not use a Post-it note. Maybe in another 10 to 15 years, texting this information will be commonplace and acceptable, but for now, I'm against that one, too.
It's tempting to think you can get away with ignoring the person in question. You can rationalize it by saying, "We both felt the same vibe at the end of the date," or, "I could tell s/he wasn't expecting to hear from me again." It doesn't matter if you think that; you owe it to both of you to have more official closure. Having been on the receiving end of that "vibe" comment (many, many years ago!), I can tell you that the other person doesn't necessarily feel the same way and might be looking forward to hearing from you.
No matter how much you hate confrontation or want to avoid hurting someone's feelings, I think it comes down to saying something either in person or over the phone. If, at the end of a date, you're absolutely positive you don't want to see the person again in a romantic context, you can say, "Thanks so much for a lovely evening. It's been great getting to know you, but I don't think this is going to go anywhere beyond the dates we've already had." You could also try something as vague as, "Maybe I'll see you around." (If you say this at a bar after just meeting, this is flirting, but if you say it after a second date, it's pretty much a euphemism for, "I don't want to see you around.")
If you can't work up the courage to say something right away, or you need time to decide if you want to see the person again, go with a phone call. (After only one or two dates, you don't need to meet up to say this in person. After four or five or six dates, you probably do.) You can use the above line about being lovely to get to know each other. If there's a specific deal breaker, you could mention that. ("I realized I won't be happy with someone who…," or, "I can't get serious with someone who….") The shorter the conversation, the better. Aim to be both friendly and direct: Friendly so you don't come off as a jerk, but direct so that there's no question as to whether or not you actually do want to go out again.
In your list of options, you don't mention email, which actually can work here — especially if most of your communication up to this point has been over email. (Again, not if we're talking five or six dates, but after one or two, I think it's okay.) Stick to the same rules as a phone call: friendly, brief and direct, and don't expect a response.
As unpleasant as this situation is, the dead-end first date is an integral part of dating and being able to handle it gracefully is a skill worth cultivating.