I like to use
git stash
This stores all uncommitted changes in the stash. If you want to discard these changes later just git stash drop
(or git stash pop
to restore them).
Though this is technically not the "proper" way to discard changes (as other answers and comments have pointed out).
SourceTree: On the top bar click on icon 'Stash', type its name and create. Then in left vertical menu you can "show" all Stash and delete in right-click menu. There is probably no other way in ST to discard all files at once.