How to store frozen broccoli after opening the package?

If you can fold the top of the bag over - rolling it several times to make everything nice and tight - you can put a large rubber band around the bag. That's what I've done! An alternative would be to fold it over two or three times and clamp it with a bulldog or similar clip (something good and strong). If you can't reuse the bag, you can put the broccoli in a ziplog bag designed for use in the freezer or (better, as more environmentally friendly) a tightly sealed plastic storage container - or two or three, in smaller portions, if the geometry works better in your freezer. ;o)