Disagreements and a lack of focus in the group

I'm started to get a little bit frustrated with certain members of the group presenting artefacts that have no relevance or coherence to what we have discussed. I am sure I am not the only person in the group who feels this way. I, along with other members have tried to express this to the people in question and although they appear to understand, continue with the same disregard. This has been occurring all the way through the project and less than a week away from the presentation deadline and I'm quite stressed about this lack of focus and also worried by the fact that some members of the group are either trying to change the product from what the rest of the group have agreed or perhaps do not understand the basic premise of the task at hand. Either way, I am finding it difficult within the group at the moment.