I was starting to get so frustrated when choosing my bobbins because of all the loose threads. I don't remember where I saw this idea but I thought I'd try it and it works beautifully.
I went to a home store (Menards, Lowes, Home Depot) and picked up some vinyl tubing. I picked up a whole lot of it (more than enough to share) for under $10. What you need to get is the following:
1/2" I.D. X 5/8" O.D. Clear Vinyl Tubing
The I.D. means inner diameter and the O.D. means outer diameter.
I found that the easiest way to cut them is to cut off about an 8-inch length. Cut a slit lengthwise then open up one of the ends and cut it into thin (under 1/4") strips. I didn't measure, I just judged by eye. They just need to fit in the bobbin to cover the thread.
Then you just wrap the plastic piece around the thread. That's it, you're done!