What consequences do you have in place for failure to be on task? Are the consistently implemented? The consequences might need to be put into place, made more serious, or enforced more strictly.
This sounds interesting! How old are they? Do they see your sessions purely as extra rehearsal time? And, do they think they know it all, or at least, do they think they know more than you? If this is the case, then I would perhaps think about levelling with them about certain areas of experience. Admit that you may not be able to play the guitar (forgive me if you can!!) but let them know with certainty that you have far more experience with lesson set-up, teaching, structure, and basically everything they need to know in order to be successful. Talk about successes you've had in other areas maybe? And if they want some of that, to simply listen...
Sorry to hear about your current 'lack of respect' troubles. The students you refer to I assume are studying for a qualification that means they have to follow and complete modules? At the start of the session do you list out clear aims of the lesson including key times and learning steps they are expected to achieve. I believe the first 2 minutes of a lesson probably lays down how the overall behaviour will be. A clear plan will help you be able to keep the detractors on track and also allow you to warn risks of not achieving aims if the class is falling behind schedule. If you can plan a positive, enjoyable experience for the final 10 mins of the lesson, missing out on this might tip the balance in your favour.