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.
+1. I’ve standardized on 3 thumb keys per hand across all my keyboards.