This handy little item is called a DAP CAP and it's available in the Home Depot paint department. It serves two purposes. #1 It can be used to clean up the excess sealant that squeezes out. #2 It fits over the end of an open tube of caulk to seal it for future use.
WARNING: The DAP CAP does not fit Sikaflex nozzles but does work on 3M brand tubes.