On SourceTree for Mac, right click the files you want to discard (in the Files in the working tree list), and choose Reset.
On SourceTree for Windows, right click the files you want to discard (in the Working Copy Changes list), and choose Discard.
On git, you’d simply do:
git reset --hard to discard changes made to versioned files;
git clean -xdf to erase new (untracked) files, including ignored ones (the x option). d is to also remove untracked directories and f to force.