how to discard thousands of files

Question

My repository initially cloned many directories I did not need, so I deleted them from my cloned repository. Now those thousands (and thousands) of files show up in my Sourcetree Unstaged files area. I can select them one by one to delete, but it very slow and taking hours. Is there a way to mass delete these from the Unstaged area, for example not using the shift key and then scrolling or clicking to the bottom to select large groups, Sourcetree hangs when doing that.

Darryl Blake · Answer

Speaking as Newbie. How do I access the command line?
Is this the Terminal section of the sourcetree?

Update: I tried the line in the terminal and it didn't work. Said, bash:syntax error near unexpected token "newline"
What ever that means, I have no clue.

UPDATE: For anyone else whom was also looking how to remove unstage files in their Sourcetree.
Select the Terminal in the top right screen area. 
Within the terminal type,

git reset --hard HEAD     hit ENTER
You should now see a % of the files being removed.

Brant Schroeder · Answer

@Lyn Hunter Welcome to the Atlassian Community.

To delete unstaged files, you can do it from the command line using

git checkout -- <file>