So, I bought this neutral silicone tube to seal a couple of cracks. The tube of silicone is still half full. How do I store it?
Stick a deck or drywall screw in the hole. Remove it from the applicator.
It will harden up. When ready to use again, yank out the drywall screw.
Remember silicones have a finite shelf life of only a year, or maybe two if stored in ideal conditions. The worst thing you can possibly do to yourself is try to use a silicone sealant that is too old. The problem isn't just it failing, but the huge mess of removing all of the faulty product so a new application has a chance! Silicones are annoyingy hard to remove.
Because of this I am very shy about reusing old silicones unless I am very sure of their provenance. It's not worth screwing up your project to save half of $5.