What do you do with left over caulk in a tube?
I wrap any adhesive tape I have on hand into a tube extending a short distance beyond the tip of the nozzle and pinch it shut at the end, then I dispense some product into the tape. This allows the product itself to seal the tube on the outside of the tube. 

When you're ready to use the tube again, yank off the tape and the dried plug and you should have a fresh nozzle.