This isn't exactly a sewing tutorial, but it has step-by-step instructions and photos, so I put it here.
Recently I had a problem using invisible thread on my vertical spool pin. Because of the nature of invisible thread, if any slack came into the thread, it would unwind, fall past the spool, and then tighten around the pin when I started sewing again. I came to this board to see if there was an answer. Unfortunately, most of the answers were for horizontal spools.
I puzzled over this and finally came up with a solution. It looks silly, but most things I think up usually do. (Just ask my wife.) Here are the steps along with photos. I use Coats and Clark thread that has the groove all the around the edge of the spool for securing your thread.
1. Cut two 3/4 inch strips from an index card, then cut them in half.
2. Place the spool on the end of each piece of index card and trace it. Shown in Sharpie for clarity.
3. Cut along the line.
4. Insert the cut edge of the index cards into the groove of the spool. In the photo it looks like the spool is simply sitting on top of the strips, so . . .
5. This is where you can laugh. It looks like the top of a propeller beanie, I know.
6. Here it is on top of my machine.
Now if the thread unwinds, it can't get past the bottom edge.