The quickest and most effective way to inactivate proteinase K is by using heat. 

As with most protein enzymes, proteinase K is most efficient within its optimum temperature of 65 ˚C. Heating proteinase K to 95 ˚C for 10 minutes will inactivate it.  However, proteinase K cannot be fully heat-inactivated. Even when heated to 95 ˚C, some amount of enzymatic activity still remains. 

To completely remove any leftover enzyme for protocols such as PCR sample preparation or nucleic acid isolation, heat treatment is followed by washing the enzyme or with the use of protease inhibitors.