put some glue or tighten bar end till it doesn’t move any more 
Superglue is fine, but everything needs to be grease free.
BTW, on an auto scooter the left grip will move in a way it won’t on a regular bike as it is part of the braking system and hard application of the brakes will tend to make the grip rotate if it is not securely glued in a way that grabbing for the clutch will never do.
When I browsed the www Gorilla glue came up as the most quoted glue to use. It is certainly better than the crap I bought labelled as motorcycle grip glue.
i always use hairspray on my grips (i changed then back and forth heated2normal all the time) and it never fails.
one tip tho. buy the cheapest hairspray you can find! sounds weird but it means it will have the most glue - bad for the hair but good for the grips
Big big hammer
+1
Try those shops that sell everything for 99p or less, the cheaper & nastier the better! 
Ohhh, that explains it. I used some expensive stuff that I stole from the wife, I’ll go buy some cheap and nasty
i use compressed air to remove and hairspray to refit!