You can also add a blank wav and just rename this file to the hitnormal sampleset you are using. You can download the official blank wav here https://up.ppy.sh/files/blank.wav
To do this add a new green line to the sliderend you wanted to silenced then make volume 5% and change the custom number to your liking (for example you change it to custom 69) then add the blank wav to your map folder and rename it to the hitnormal sampleset with the number you choosed (for example you used a soft sampleset then its "soft-hitnormal69.wav").