You would need to be very sure, and aware of relevant company procedure and legislation. Assuming it's permissible, then it depends on the situation: if...
- it becomes evident that the candidate has misled you (but consider drawing their attention to the apparent contradiction, as there may be a reasonable explanation).
- the candidate consistently demonstrates a lack of knowledge which they claim to have (but if possible attempt to give them an opportunity to demonstrate to you what they do know).
- it is clear that the person in front of you did not intend to apply for the job you're interviewing for (but check this with them).
...then stopping may be best for everyone, especially if the interview is an hour or longer. In each case, if you make it clear what the job requires and that they appear to be struggling, it will give them the opportunity to address the problem (possibly even by agreeing with you that the job is not for them!). It's a good idea to be prepared for this eventuality - make sure it's clear who, from the interviewers, will be responsible for making this call, and preferably structure your interview so there's a reasonable point to take a break in case you need to confer over a change in approach.
But if it might just be that...
- the candidate begins the interview nervously, and is having mind-blanks,
- the initial questions are acknowledged weaknesses and the candidate's strengths lie in other areas,
- you think they won't get the job because you've already interviewed another candidate,
... then you should continue, and look for the best ways to help the candidate demonstrate what they have to offer. Other candidates might pull out, the person in front of you might astonish you later in the interview. If you were surprised by how badly they appear to be doing, there must be a reason. If nothing else, proceeding will allow you to be as sure as you can be that you gave the candidate the best chance... maybe you need to read applications/CVs more closely, for instance?