It would depend on your role in the interview.
In many organizations, a candidate is interviewed in 3 or 4 stages. You have an interview with the intended supervisor of the candidate, and then a "team" interview with the intended coworkers, a quick one-on-one with a senior manager, and perhaps even a separate interview with an HR specialist to verify credentials.
If you are the sole decision-maker about the candidate, then I think it would only be fair to tell them, "Your experience doesn't meet our requirements for this position. Thank you for coming in, today. We wish you the best for your future." Then politely end the interview.
It may seem harsh, but it's even worse to let someone who is obviously unqualified think, "I might have a shot!" for the next few weeks and then wonder why you never got back to them. It may even be discouraging them from pursuing other opportunities if they are "holding out" for the job you interviewed them for.
If you are not the decision-maker, but you have an input into the decision, then you have to complete the task assigned. Go through all the questions you've prepared, and take note of the answers. At the end, if the candidates asks how they did, then you have to respond, "I'm not the sole decision-maker for the position. We have to complete the interview process before a decision will be made." and leave it at that.
When you report to your colleagues, don't mince words. Tell your them directly and honestly what your impression was. I saw an entire division of a major cable company go down in flames because the president was trying to be "polite" about the qualifications of a candidate for the vice-president that ended up in charge of our division. (It was WAY above my pay grade at the time, but it was such a spectacular failure that it became famous in that business.)
If you have any influence at all, encourage your team to tell the candidate as quickly as possible that they weren't selected. It's not fair to just "forget" about them. If you get as far as bringing them in for an interview, it's very unprofessional to just not take their calls. You're a decision maker, make the decision, stand by it, and don't hide from it.