I usually do it with a dremel. Saw it off as close as you're comfortable, then carefully sand it the rest of the way. might try hand-sanding it with a high grit sandpaper right at the end. Odds are you might mess up a bit of the paint on the bottom of it if it's already painted. I've never tried doing it with a Hot Toys sculpt, only unpainted ones...
For the neck adapter, I usually pick up a cheap head off eBay that comes with one in it, and then pry it out. Drill out the head wide enough for the adapter to slide in, glue it, and you're good to go.