Get a big pair of pliers....
and close the "hook" so it won't come out... period. However, you should have a better attachment than an open hook... that's kind of cheesy. How about changing out the hook deal with a more permanent connector, like a shackle or marine eye/clevis pin. When I trailered a boat I only removed the shrouds from the mast if the boat wasn't going to be used for a long time. I had quick connect pins and adjusters on the chainplate ends. After dropping the mast I coiled up the shrouds and secured them to the mast or cabin top with some extra line.