The default keybinding for killing a buffer should be SPC b k. That is what I use to close the scratch buffer. 

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 !GameWarrior@lemmy.world 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.