The default keybinding for killing a buffer should be SPC b k. That is what I use to close the scratch buffer. 
this post was submitted on 21 Jun 2023
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If you choose SPC < to bring up the window that shows all open buffers, you can choose it and hit SPC b k (buffer, kill) as [email protected] suggested. You can also choose SPC b d (buffer, delete) which is the same as SPC b k, or just hit SPC b and look at the options available to you.
I use Ctrl-w-c (it's on the same side of the keyboard) or space-b-k. I'm sure you could you set your own convienient shortcut, it's Emacs after all.