I found the most wonderful way to keep my bobbins from unreeling and causing chaos in my bobbin holder at Sew Many Ways!
First you need a length of flexible tubing (found in the plumbing aisle of the hardware store), a Sharpie marker, and either an old sharp pair of scissors or a utility knife.
Measure the inside of your bobbin. I did this by lining it up next to the tubing.
Mark the inside.
Then cut the tubing to the desired length.
After it's the width you want then cut through the tube on one side so you can slip it over the bobbin.
Slip it over the bobbin and you're done! I like these because they allow me to still see the color of the thread and they keep the threads all inside.
My bobbin case has never looked better!