Quoted from Ballypin:The easiest way to remove them is to use a lighter and put the flame near the open end of the bell for about 5 to 10 sections. This will heat up the bell and the surrounding plastic so that you can use a pair of needle nose pliers to grasp the hot bell and remove it from the plastic.
I bookmarked this trick a while ago. Tried it since and it works great.
Perfect solution for us non-smokers who don't own a lighter 
https://pinside.com/pinball/forum/topic/eight-ball-deluxe-owner-club-guests-welcome-as-well/page/34#post-4258662
