The only thing that will make an engine run away like that is something that holds the throttle slide in the carb open. A couple of common scenarios occur,
Throttle tube sticking due to a fall or grip rubbing on end of handle bar. You may have noticed a lot of riders use "bar ends". Those things that sit in the hole at the end of the bar and require you to cut open the ends of your grips. These are popular for two reasons, First is you tend not to rip up grips as quickly. Second, in a fall the bar end takes the hit and you are less likely to mash the throttle tube further into the bar or even damage it so it doesn't turn freely. I highly recommend you get a set of bar ends and open the end of your grips.
The other thing that is common with any bike is the cable getting pulled out of either the throttle housing at the top or, in the case of the Beta, the "noodle" that comes out of the top of the carb. Here's where it's important to get into the habit of before you start the bike twist the throttle and let it go and listen for the "thwock" sound that comes from the carb as the slide hits the idle adjust bolt. If you don't hear it DON'T start it. Fix it by making sure the grip is loose and the cable hasn't pulled out. Make it a habit. I don't even think about it anymore I just do it by reflex and it's kept me from the full throttle start a few times. It also has the tendency to seat a cable that isn't fully seated if you twist to the full throttle position.
Now as for not being able to kill a runaway engine at full throttle that is a fairly common experience. At that speed there is more than enough heat and free radical chemical compounds to keep an engine going without any spark. As for pulling the plug wire, I did it once. Never again, I actually felt my heart stop until the engine wound down. Scary as S***. As the wise folks above said stuff something over the exhaust. A glove a boot whatever. It will usually kill or slow down the engine enough for the kill switch to work again.