Unless you root your phone you will be unable to stop the constant pop up notifications to update your phone.
I was in the same boat, Heck I even created my own thread JUST like yours titled "Enough is Enough 4.3" in the Virgin Mobile forums.
After about a week of getting the update notification every 5 or 10 minutes 24 hours a day, I finally got fed up and updated...
Really can't say I regret it as the phone is a LOT faster for me. I personally have not experienced any of the problems others have stated here on the boards. Even then, If you do experience the most common bugs..ie...old downloads...flash notifications, etc...These all are EASILY fixed within a minute by toggling off within menus.