This afternoon I went to change a space that was previously private, accessible only to two people, into a public one. I went onto the Permission Summary screen (cf. attached screenshot).

I started double-clicking each of the boxes in the Registered Users row (fiddly! involves much clicking!). Once I had done this I clicked the delete button against the first user to whom permission had been explicitly granted (me), since this was now redundant. After clicking the button to confirm I did want to delete the row, the row had disappeared correctly, but the Registered Users had reverted to being red-crossed.