So sorry to hear that. 😟 I know how frustrated I was by this ridiculously irritating software.
All I know is you have to be super quick!!!!, on account of how quickly it pops back to being active. I had the finder window already open with Megabackup in it and ready to delete. I closed it (make sure you get both items that show Megabackup in the name--there is one that pops back open and one that doesn't, don't remember which is which, but you will learn from trial and error if you haven't already--close the one that does not pop back open first, then the one that does) from the activity monitor, immediately switched from that to finder, by clicking on the dock icon, picked up Megabackup and dropped him in the trash. He didn't even know what hit him 😉 and I didn't even need to put in my admin password!!! 🙂 It worked the first time I did it this way, but I presume it is because of the fact I had it all set up and ready to go, and I wasted no time doing it. I cannot stress to you enough how quickly I went through these steps. After all the frustration, it almost felt too easy!!! But ahhhhh, the best part? No more annoying popups and my productivity has returned to well... productive.
If you do not have success with this, then I suggest a trip to the Geek Squad or some other repair facility...