If I want to discard several changes, I have to perform two separate steps:

1. Highlight only tracked files and choose "Discard"
2. Highlight untracked files and choose "Remove"

I think untracked files should allow "Discard" which would delete the file from disk. This way, I could highlight all files and "Discard" to clean up my working copy.

The user already expects that "Discard" is a destructive operation, so the fact that the file is untracked shouldn't prevent SourceTree from deleting it.