If you do attempt to sabotage his return, it is likely that he would rely on all of his significantly powerful minions to shame you for making that choice (engineered consequence of his design, hint hint), plus doing so doesn't actually STOP him from returning, it just makes his goal slightly harder. The gamble vs the payoff (convincing you, the one being that he has zero ability to control directly (save for the elder gods, and I guess Seren) and the overseer of the planet he is attempting to gain control of, and the one being who is apparently able to survive being harmed by an elder god without being shattered into your component pieces) is actually a more vital part of his plan than a lot of other things. Turning the choice into a zero sum game for you is pretty good odds for him. I believe that this is also why the decision to allow him to enter your body is impressed upon so vigorously by the dialogue choices (and why they would specifically create a situation that gives you that choice, with a clear incentive for making it in his favor with no readily apparent reason why you shouldn't do it), and why it is specifically noted in your quest journal. It might actually have some sort of consequence later on. But like he says, you have to live with your choices, and move forward regardless of the outcome, so even if this does result in a bad consequence later on, you'll probably have some means to overcome it, as difficult as it may be.